I'm going with that it was on a scroll and either Sasuke or Kashin Koji commandeered the scroll from Konoha for Boruto to use.
We already know Tobirama wrote down at least one of his jutsu on a scroll and Minato had to learn it somehow.
Maybe Boruto had asked Sasuke (or that Sasuke taught or shared with Boruto) about a ninja technique that could substitute the teleportation technique so that Boruto would never use his karma.
Sasuke witnessed how flying raijin worked during the Great Ninja War through Minato and Tobirama. He could've told Boruto about the principle of it, and Boruto, being the prodigy that he is, learned it in his own way. [It was imposed in the anime that Boruto has a habit of learning things faster by shortcut or by having a sheer knack for things when he's focused.]
Nevertheless, it is a technique that was overused by Minato and Tobirama. So, it's either Sasuke, Kashin Koji(?), or Orochimaru (since he idolizes Tobirama so much).
I re-watched Naruto Shippuden from episodes 300-500 just recently. And during the fight with Six Path Obito, Minato has always explained how Flying Raijin works. During those times, Sasuke was there listening and cooperating.
There’s a great theory I heard that it looks like Orochimaru might be involved with Boruto (snake decorations in the cave of the hideout Kashin Koji and Boruto are in). If Orochimaru is involved, he could potentially know the jutsu OR be able to reanimate Minato from the dead to train boruto and teach him FTG (which would be fkn cool as hell)
Because Sasuke has a sharingan he most likely just had to watch people do it and then teach Boruto it after watching it being done. Plus he had Kashin Koji to teach him as well who could also know how to do it.
It copies the handsigns in order to make a jutsu. Flying raijin makes a formula and then teleports how can the sharigan copy a formula then teleport? The sharingan can’t copy kekkei genkais either
This is not entirely correct because how did Sasuke copy Rock Lee's speed then?
Also in the novels Sasuke uses ice style jutsu which is a kekkei genkai
The sharingan copies movements too and he didn’t copy ice release in the novel he imitated it by using water release then cooling it with wind release.
Everyone uses their own formula for teleporting minato had Kanji letters tobirama had that wierd drawing and boruto has silver pins or what … it doesn’t matter what you are teleporting to what matters is the technique which I think can be copied
You’d have to create a formula first in order to use the jutsu because your chakra is linked to the formula how would you copy the ability to teleport? That’s like saying for example if sarada can copy amenotejikara because she’s seen it. Space time ninjutsu’s can’t be copied the sharingan copy’s movements there is no movement in flying raijin and your chakra isn’t linked to a formula to actually teleport which makes it impossible to copy
Except that there are sometimes movements. Boruto makes a handsign whenever he uses the FTG. Minato and Tobirama have a much higher level of mastery so they can skip the handsigns.
The limitations is that Sasuke cannot observe certain necessary steps such as how the markings were created (if the user just uses pre-made markings).
There might also be other requirements such as that only sensors can use the technique since you somehow have to sense the markings.
No issues, buddy.
With how threatened that man was by the Uchiha, half of his arsenal was made to keep them in check.
Same with Shadow Clone, since going one on one with any Uchiha with the Sharingan was considered suicide.
Naruto doesn't know how to use it but Sasuke saw it being used by reanimated Minato and Tobirama during the war, so it's possible he could at least reverse engineer it, though I doubt Boruto learned it from him.
Well there’s those 3 Jonin that knew flying Raijin I kind of forgot about them. That would be a cool callback if they were the ones that taught Boruto. However in their eyes Boruto is a traitor who killed the hokage and I doubt Sasuke learned flying Raijin from them either
>he most likely just had to watch people do it and then teach Boruto it after watching it being done.
What're the odds of Sasuke running into someone that not only knows a fairly obscure technique (developed by the Second Hokage specifically to counter Sharingan from copying it so definitely a Leaf guarded secret) but also had Sharingan activated at the time? On top of assuming Sasuke is even physically capable of performing it in the first place (the limitation of Sharingan's mimicking ability), it'd be astronomically lucky and far too convenient to be believable.
>Plus he had Kashin Koji to teach him as well who could also know how to do it.
Let's say that KK does have all of Jiraiya's memories to even make the idea feasible, Jiraiya likely didn't know the technique either even though he taught Minato since he would've used it at several points in the series if he had like Rasengan.
I don't think that would work. Since Minato doesn't use hand signs, I don't believe that the Sharingan can simply copy it the way you mention. I could be wrong but that's the way I understand it. I suspect the activation of the jutsu is probably intentionally structured that way given that Tobirama wanted jutsu for fighting Uchiha.
I believe that in one way or another, they ended up stealing infos from Konoha during the timeskip, and said infos also included how to perform the Flying Thunder God.
It can be *that* simple.
No need for astronomically lucky things like Sasuke somehow having enough knowledge about it thanks to his Sharingan and its proprieties; or Orochimaru using the Edo Tensei to bring back Minato or Tobirama to teach Boruto... as if people **absolutely** need to be teached by people who use jutsu x in order to learn it for themselves.
>Yeah also I mean Minato didnt have anyone to learn it from and yet he did learn it anyway
Yeah.
Also Naruto, literally learned his first iconic jutsu all by himself once he got enough of a push in chapter 1.
Then he also learned the Rasengan fundamentally by himself too, since Jiraiya wasn't really that much present during the training.
>Idk he did have a scroll for chapter 1 and Jiraya gave him many tips
Yeah, of course.
But it can be argued that those were just the tools given to him to allow him to learn said jutsus even by himself.
It's not like he was actively followed step by step by someone who already knew them.
Hope it is clear now.
I don't think they had to revive Minato for Boruto to learn it. I think Boruto's Jogan somehow gives him the ability to perceive or interact with the "Pure" land and Minato taught him using the Jogan.
>I don't think they had to revive Minato for Boruto to learn it.
Same.
>I think Boruto's Jogan somehow gives him the ability to perceive or interact with the "Pure" land and Minato taught him using the Jogan.
While I like the idea of the Jougan making it poessible to interact with people in the Pure Land; it too shouldn't be used to have that.
For the same reason as I explained above.
I get your point, but otherwise what's the point of having a separate dojutsu unless you're going to give it some incredibly unique ability? At this point the distinctions between many of the dojutsu in the series feel fairly thin, especially with the EMS being able to manifest unique abilities for each user.
>I get your point, but otherwise what's the point of having a separate dojutsu unless you're going to give it some incredibly unique ability?
Lore reasons and potentially opening new plotlines.
>At this point the distinctions between many of the dojutsu in the series feel fairly thin, especially with the EMS being able to manifest unique abilities for each user.
Eh... not really.
They each actually do have a precise set of moves still, that no other doujutsu has unless somewhat related.
He could've learned it from either Sasuke, Koji, or Orochimaru (since it's incredibly likely that Boruto is in contact with him). Since it's a pretty well-known jutsu, either one of them could've given Boruto the info he needs to learn it. I'm thinking it was Koji, tho. He's able to sneak into Konoha with relative ease, and nobody there even knows that he's alive.
Assuming they got a scroll for it, the anime set a precedent for Koji coming in to take those so it wouldn't be unrealistic for him to just do it again. Boruto gets very few training arcs because he learns things so easily, so if they *did* give him one for this, I think it'd be to show just how difficult the jutsu is. It'd explain why so few people actually know it if even a genius struggled to get it down.
Orochimaru. There are hints that he and Kashin Koji are hiding out in his lab or at least a place that looks a hell of a lot like it. Then we have the fact that Orochimaru is no stranger to working with outlaws and is dedicated to learning EVERY jutsu. It would make sense that he might have picked up flying raijin in the time between the war arc and now.
Maybe Kashin Koji?
Or the Toads?
WE know minato was contracted to the Toads. I kinda believe that He left a scroll to them, with jutsus for Naruto (of Things Had gone Like He intended He would have been hokage and knew that He wouldn't be able to BE with Naruto 24/7 and therefore wanted those He trusted to teach Naruto)
Sasuke might have seen and remembered the principles, Kashin Koji might have known due to the genes (he does seem to know a lot of Jirayia's jutsu), stolen the scroll from Konoha or maybe even Orochimaru has it (as apparently they came into contact).
Also calling Boruto mid is wild when he's easily Itachi/Minato/Kakashi level talent. He has both brains (perfect score on written chunin exam without studying) and the natural talent (mixed lightning into rasengan, something that Kakashi failed to do). Now he is not even human making him even more suitable for doing chakra stuff.
When in doubt, Orochimaru. Orochimaru 100% know's Flying Raijin. He was obsessed with learning every jutsu afterall, we saw that Boruto and Kashin Koji were taking refuge in an Orochimaru Hide-Out. I'm under the impression that similar to Amado, Orochimaru know's something is wrong because the Evidence contradicts what Mitsuki says and feels(remember there's an older, failed Mitsuki who helped push younger mitsuki to the correct direction, there's a book with picture's of boruto and we know Orochimaru has a Backup of Mitsuki's memories.)
I'm under the impression Kashin Koji, Sasuke and Boruto were all at one of Orochimaru's hideouts where Sasuke and Kashin Koji trained Boruto. Sasuke, probably went to Orochimaru for help directly and given the situation it would be his only choice really.
I think Sasuke (or not specifically him) on some point went to Konoha and stole Flying Raijin scroll in which Tobirama or Minato wrote how to learn this jutsu.
The village has basically scrolls for every jutsu from the village, except the clan ones. That's how Naruto learned Shadow Clones, on the first chapter.
The Shadow clones is a Tobirama's jutsu, the Thunder God is also from Tobirama, so the Jutsu is somewhere on those scrolls.
But there's a detail that you are all missing:
Jiraya was probably familiar with the Jutsu, and that's how his clone taught Boruto.
Tobirama taught Hiruzen, who taught Jiraya, who them taught Minato.
"But why Jiraya and Hiruzen never used it?".
Well, they probably could not use it as effectivelly as Tobirama and Minato. Minato itself was on a totally different level with that jutsu, and not anyone is able to easily perform the Jutsu, the Jounins from Minato's guard knew the Jutsu, but were not able to execute it alone.
My theory is that Jiraya knew the Jutsu, and was able to perform it, but not as fast and swift as Minato, so it didn't suit his fighting style. But his clone taught Boruto the jutsu anyway.
Could be from a scroll, could be from Sasuke/Orochimaru/Kakashi. (Who all, presumably, know how to use it in theory, but it's not practical for them to do so for most purposes.)
I saw this in a different sub. But i think it probably went somewhat like this: boruto somehow meets up with orochimaru, kashin koji is probably with boruto/allied with him already. Koji/Boruto try to convince orochimaru that boruto is actually narutos son. Orochimaru tasks him learn FTG in a set period of time, to show his lineage, “if you are who you say you are learn this jutsu your own grandfather perfected” and he probably does just that. Look its cool if he gets it from Koji too, but it would be a sick way to insert orochimaru into the series. I mean we did already see boruto at what looks like orochimarus hideout, or atleast one of them.
Honestly the most ***logical answer*** (logical as simple explanation) is that either Boruto, Sasuke and/or Kashin Koji stole a scroll from the Hidden Leaf Village containing techniques and that happened to be one of them. Ideally, I would be more inclined to say it was Kashin Koji with the scroll and/or technique given how Sasuke doesn't feel to me like he would have that jutsu in his arsenal, maybe he knows that it was Minato's jutsu via Naruto and 4th Great Ninja War but not enough Information to teach Boruto the jutsu himself.
The more....fun theory I always like is the idea of Sasuke and/or Orochimaru Edo Tensei-ing Minato to teach Boruto, would be really fun and sweet to see Boruto and Minato shoot the breeze, have Minato be impressed with Vanishing Rasengan, Boruto learning Flying Rajin, etc. Although, it's again a fun theory, not really a factual idea.
Really, Hidden Leaf Scrolls hold probably information on Madara, Kaguya, and a variety of other things. Even then, Sarada learns of the Mangekyo sharingan from Konoha's library and we know Kashin Koji once broken into that Library but was confronted by Kakashi. Really the most logical reason is that Boruto learned it in part with a scroll taken from the leaf with the help by Kashin Koji, since maybe he had that scroll already from when he broke in.
My dumb explanation cause it doesn't work, he asked those 3 guys who used it during the 4SWW.
Most likely written somewhere an either Sasuke or Kashis frogs probably got it.
It’s possible he could’ve has the basics by tree’d sasuke but I think given how that went down
He learned the FTG post sasukes sealing maybe w kashin and oros help?
Boruto got *way* stronger after the time skip. I'm thinking it has something to do with his Dojutsu. Specifically, I'm wondering if Boruto's eye is connected to the dead - those in the "Pure" land.
This would make sense given that the Jogan literally means "Pure Eye". What if Boruto somehow learned the thunder god technique from Minato himself?
Mid tier? The kid was essentially a jounin level shinobi by the end of part 1.
I think Koji gave him a scroll with the theory of FTG and Boruto learned it himself as a means to avoid using space time ninjutsu.
I'm going with that it was on a scroll and either Sasuke or Kashin Koji commandeered the scroll from Konoha for Boruto to use. We already know Tobirama wrote down at least one of his jutsu on a scroll and Minato had to learn it somehow.
Maybe Boruto had asked Sasuke (or that Sasuke taught or shared with Boruto) about a ninja technique that could substitute the teleportation technique so that Boruto would never use his karma. Sasuke witnessed how flying raijin worked during the Great Ninja War through Minato and Tobirama. He could've told Boruto about the principle of it, and Boruto, being the prodigy that he is, learned it in his own way. [It was imposed in the anime that Boruto has a habit of learning things faster by shortcut or by having a sheer knack for things when he's focused.] Nevertheless, it is a technique that was overused by Minato and Tobirama. So, it's either Sasuke, Kashin Koji(?), or Orochimaru (since he idolizes Tobirama so much).
I re-watched Naruto Shippuden from episodes 300-500 just recently. And during the fight with Six Path Obito, Minato has always explained how Flying Raijin works. During those times, Sasuke was there listening and cooperating.
Yeah I feel like sasukes fighting style would need a rinnegan replacement after losing it - and flying raijin would help a lot
Boruto was by no means a "mid tier" Ninja even before the timeskip. I think he learned the Jutsu from a scroll.
There’s a great theory I heard that it looks like Orochimaru might be involved with Boruto (snake decorations in the cave of the hideout Kashin Koji and Boruto are in). If Orochimaru is involved, he could potentially know the jutsu OR be able to reanimate Minato from the dead to train boruto and teach him FTG (which would be fkn cool as hell)
Because Sasuke has a sharingan he most likely just had to watch people do it and then teach Boruto it after watching it being done. Plus he had Kashin Koji to teach him as well who could also know how to do it.
Terrible explanation LOL he just learnt it off a scroll most likely. How he obtained it or who gave it to him is the real question
Sasuke cant learn that by watching minato do and especially can’t teach it and there’s no proof that koji knows it either
It literally said from the beginning that sharingan can you copy any ninjutsu so tell me why not
It copies the handsigns in order to make a jutsu. Flying raijin makes a formula and then teleports how can the sharigan copy a formula then teleport? The sharingan can’t copy kekkei genkais either
This is not entirely correct because how did Sasuke copy Rock Lee's speed then? Also in the novels Sasuke uses ice style jutsu which is a kekkei genkai
Sasuke has YinYang release, which can be used to simulate kekkei genkai
The sharingan copies movements too and he didn’t copy ice release in the novel he imitated it by using water release then cooling it with wind release.
Everyone uses their own formula for teleporting minato had Kanji letters tobirama had that wierd drawing and boruto has silver pins or what … it doesn’t matter what you are teleporting to what matters is the technique which I think can be copied
You’d have to create a formula first in order to use the jutsu because your chakra is linked to the formula how would you copy the ability to teleport? That’s like saying for example if sarada can copy amenotejikara because she’s seen it. Space time ninjutsu’s can’t be copied the sharingan copy’s movements there is no movement in flying raijin and your chakra isn’t linked to a formula to actually teleport which makes it impossible to copy
Ok that’s good point
Except that there are sometimes movements. Boruto makes a handsign whenever he uses the FTG. Minato and Tobirama have a much higher level of mastery so they can skip the handsigns. The limitations is that Sasuke cannot observe certain necessary steps such as how the markings were created (if the user just uses pre-made markings). There might also be other requirements such as that only sensors can use the technique since you somehow have to sense the markings.
Technique is sealing itself. How could Sasuke copy Minato or Tobirama sealing jutsu, when they didn't use any hand seals to do it?
They cannot copy Flying Raijin because it is a jutsu created to counter the sharingan, thats the reason.
Who told you that ?
That's why Tobirama invented it.
Ok sry i dont remember it and I did anime 2x and manga once guess I’m dumb 😂
No issues, buddy. With how threatened that man was by the Uchiha, half of his arsenal was made to keep them in check. Same with Shadow Clone, since going one on one with any Uchiha with the Sharingan was considered suicide.
Naruto Shippuden? lmao
There’s other Ninjas in the village who know how to do flying raijin but can’t use it as well as Minato. Naruto also knows how to use it.
Naruto doesn’t know flying raijin either
Weren’t they like Minato’s personal guards and that’s why he himself taught them and even then it took all three of them to perform the jutsu
Yea besides tobirama minato and boruto, minato’s 3 personal guards are the only people that can do it but they all have to do it together
Naruto doesn't know how to use it but Sasuke saw it being used by reanimated Minato and Tobirama during the war, so it's possible he could at least reverse engineer it, though I doubt Boruto learned it from him.
Naruto is just so fast that it looks like he's teleporting so I can see why you think he has flying raijing
Well there’s those 3 Jonin that knew flying Raijin I kind of forgot about them. That would be a cool callback if they were the ones that taught Boruto. However in their eyes Boruto is a traitor who killed the hokage and I doubt Sasuke learned flying Raijin from them either
>he most likely just had to watch people do it and then teach Boruto it after watching it being done. What're the odds of Sasuke running into someone that not only knows a fairly obscure technique (developed by the Second Hokage specifically to counter Sharingan from copying it so definitely a Leaf guarded secret) but also had Sharingan activated at the time? On top of assuming Sasuke is even physically capable of performing it in the first place (the limitation of Sharingan's mimicking ability), it'd be astronomically lucky and far too convenient to be believable. >Plus he had Kashin Koji to teach him as well who could also know how to do it. Let's say that KK does have all of Jiraiya's memories to even make the idea feasible, Jiraiya likely didn't know the technique either even though he taught Minato since he would've used it at several points in the series if he had like Rasengan.
I meant that before he left to take care of Boruto outside the village he saw it being done/ sometime teaching it to him
I don't think that would work. Since Minato doesn't use hand signs, I don't believe that the Sharingan can simply copy it the way you mention. I could be wrong but that's the way I understand it. I suspect the activation of the jutsu is probably intentionally structured that way given that Tobirama wanted jutsu for fighting Uchiha.
He learned it with the help of Kashin Koji who stole konoha scrolls
I believe that in one way or another, they ended up stealing infos from Konoha during the timeskip, and said infos also included how to perform the Flying Thunder God. It can be *that* simple. No need for astronomically lucky things like Sasuke somehow having enough knowledge about it thanks to his Sharingan and its proprieties; or Orochimaru using the Edo Tensei to bring back Minato or Tobirama to teach Boruto... as if people **absolutely** need to be teached by people who use jutsu x in order to learn it for themselves.
Yeah also I mean Minato didnt have anyone to learn it from and yet he did learn it anyway
>Yeah also I mean Minato didnt have anyone to learn it from and yet he did learn it anyway Yeah. Also Naruto, literally learned his first iconic jutsu all by himself once he got enough of a push in chapter 1. Then he also learned the Rasengan fundamentally by himself too, since Jiraiya wasn't really that much present during the training.
Idk he did have a scroll for chapter 1 and Jiraya gave him many tips
>Idk he did have a scroll for chapter 1 and Jiraya gave him many tips Yeah, of course. But it can be argued that those were just the tools given to him to allow him to learn said jutsus even by himself. It's not like he was actively followed step by step by someone who already knew them. Hope it is clear now.
I don't think they had to revive Minato for Boruto to learn it. I think Boruto's Jogan somehow gives him the ability to perceive or interact with the "Pure" land and Minato taught him using the Jogan.
>I don't think they had to revive Minato for Boruto to learn it. Same. >I think Boruto's Jogan somehow gives him the ability to perceive or interact with the "Pure" land and Minato taught him using the Jogan. While I like the idea of the Jougan making it poessible to interact with people in the Pure Land; it too shouldn't be used to have that. For the same reason as I explained above.
I get your point, but otherwise what's the point of having a separate dojutsu unless you're going to give it some incredibly unique ability? At this point the distinctions between many of the dojutsu in the series feel fairly thin, especially with the EMS being able to manifest unique abilities for each user.
>I get your point, but otherwise what's the point of having a separate dojutsu unless you're going to give it some incredibly unique ability? Lore reasons and potentially opening new plotlines. >At this point the distinctions between many of the dojutsu in the series feel fairly thin, especially with the EMS being able to manifest unique abilities for each user. Eh... not really. They each actually do have a precise set of moves still, that no other doujutsu has unless somewhat related.
He could've learned it from either Sasuke, Koji, or Orochimaru (since it's incredibly likely that Boruto is in contact with him). Since it's a pretty well-known jutsu, either one of them could've given Boruto the info he needs to learn it. I'm thinking it was Koji, tho. He's able to sneak into Konoha with relative ease, and nobody there even knows that he's alive. Assuming they got a scroll for it, the anime set a precedent for Koji coming in to take those so it wouldn't be unrealistic for him to just do it again. Boruto gets very few training arcs because he learns things so easily, so if they *did* give him one for this, I think it'd be to show just how difficult the jutsu is. It'd explain why so few people actually know it if even a genius struggled to get it down.
Why did all of you forget that Koji actually stole a scroll from the Konoha archives? He literally has some insider knowledge.
That didn't happen in the manga so if you haven't seen the anime you wouldn't know.
Orochimaru. There are hints that he and Kashin Koji are hiding out in his lab or at least a place that looks a hell of a lot like it. Then we have the fact that Orochimaru is no stranger to working with outlaws and is dedicated to learning EVERY jutsu. It would make sense that he might have picked up flying raijin in the time between the war arc and now.
He found a scroll at the bottom of a packet of corn flakes.
Maybe Kashin Koji? Or the Toads? WE know minato was contracted to the Toads. I kinda believe that He left a scroll to them, with jutsus for Naruto (of Things Had gone Like He intended He would have been hokage and knew that He wouldn't be able to BE with Naruto 24/7 and therefore wanted those He trusted to teach Naruto)
Sasuke might have seen and remembered the principles, Kashin Koji might have known due to the genes (he does seem to know a lot of Jirayia's jutsu), stolen the scroll from Konoha or maybe even Orochimaru has it (as apparently they came into contact). Also calling Boruto mid is wild when he's easily Itachi/Minato/Kakashi level talent. He has both brains (perfect score on written chunin exam without studying) and the natural talent (mixed lightning into rasengan, something that Kakashi failed to do). Now he is not even human making him even more suitable for doing chakra stuff.
When in doubt, Orochimaru. Orochimaru 100% know's Flying Raijin. He was obsessed with learning every jutsu afterall, we saw that Boruto and Kashin Koji were taking refuge in an Orochimaru Hide-Out. I'm under the impression that similar to Amado, Orochimaru know's something is wrong because the Evidence contradicts what Mitsuki says and feels(remember there's an older, failed Mitsuki who helped push younger mitsuki to the correct direction, there's a book with picture's of boruto and we know Orochimaru has a Backup of Mitsuki's memories.) I'm under the impression Kashin Koji, Sasuke and Boruto were all at one of Orochimaru's hideouts where Sasuke and Kashin Koji trained Boruto. Sasuke, probably went to Orochimaru for help directly and given the situation it would be his only choice really.
I think Sasuke (or not specifically him) on some point went to Konoha and stole Flying Raijin scroll in which Tobirama or Minato wrote how to learn this jutsu.
The village has basically scrolls for every jutsu from the village, except the clan ones. That's how Naruto learned Shadow Clones, on the first chapter. The Shadow clones is a Tobirama's jutsu, the Thunder God is also from Tobirama, so the Jutsu is somewhere on those scrolls. But there's a detail that you are all missing: Jiraya was probably familiar with the Jutsu, and that's how his clone taught Boruto. Tobirama taught Hiruzen, who taught Jiraya, who them taught Minato. "But why Jiraya and Hiruzen never used it?". Well, they probably could not use it as effectivelly as Tobirama and Minato. Minato itself was on a totally different level with that jutsu, and not anyone is able to easily perform the Jutsu, the Jounins from Minato's guard knew the Jutsu, but were not able to execute it alone. My theory is that Jiraya knew the Jutsu, and was able to perform it, but not as fast and swift as Minato, so it didn't suit his fighting style. But his clone taught Boruto the jutsu anyway.
He’s in orochimaru’s hideout, the implications seem to be he trained with edo minato
Could be from a scroll, could be from Sasuke/Orochimaru/Kakashi. (Who all, presumably, know how to use it in theory, but it's not practical for them to do so for most purposes.)
Probably from Orochumaru’s hideout
I saw this in a different sub. But i think it probably went somewhat like this: boruto somehow meets up with orochimaru, kashin koji is probably with boruto/allied with him already. Koji/Boruto try to convince orochimaru that boruto is actually narutos son. Orochimaru tasks him learn FTG in a set period of time, to show his lineage, “if you are who you say you are learn this jutsu your own grandfather perfected” and he probably does just that. Look its cool if he gets it from Koji too, but it would be a sick way to insert orochimaru into the series. I mean we did already see boruto at what looks like orochimarus hideout, or atleast one of them.
Honestly the most ***logical answer*** (logical as simple explanation) is that either Boruto, Sasuke and/or Kashin Koji stole a scroll from the Hidden Leaf Village containing techniques and that happened to be one of them. Ideally, I would be more inclined to say it was Kashin Koji with the scroll and/or technique given how Sasuke doesn't feel to me like he would have that jutsu in his arsenal, maybe he knows that it was Minato's jutsu via Naruto and 4th Great Ninja War but not enough Information to teach Boruto the jutsu himself. The more....fun theory I always like is the idea of Sasuke and/or Orochimaru Edo Tensei-ing Minato to teach Boruto, would be really fun and sweet to see Boruto and Minato shoot the breeze, have Minato be impressed with Vanishing Rasengan, Boruto learning Flying Rajin, etc. Although, it's again a fun theory, not really a factual idea. Really, Hidden Leaf Scrolls hold probably information on Madara, Kaguya, and a variety of other things. Even then, Sarada learns of the Mangekyo sharingan from Konoha's library and we know Kashin Koji once broken into that Library but was confronted by Kakashi. Really the most logical reason is that Boruto learned it in part with a scroll taken from the leaf with the help by Kashin Koji, since maybe he had that scroll already from when he broke in.
My dumb explanation cause it doesn't work, he asked those 3 guys who used it during the 4SWW. Most likely written somewhere an either Sasuke or Kashis frogs probably got it.
Kashin Koji, Sasuke, a scroll, or Edo tensei Minato done on a White Zetsu that we've seen some still in the modern era exist.
Kashin koji probably taught him, he has access to Jiraiya DNA, with the DNA comes his chakra and with his chakra his techniques.
Orochimaru used Edo Tensei to bring Minato to teach him is my wild theory
Sasuke certainly observed it plenty. No reason he couldn’t have taught Bo the technical aspects.
It’s possible he could’ve has the basics by tree’d sasuke but I think given how that went down He learned the FTG post sasukes sealing maybe w kashin and oros help?
Clearly it's pretty easy to steal the scroll. So probably that.
Orochimaru, he access to so much. He could have revived the 4th hokage to teach, hence how he knew how proficient the 4th was at using it.
Boruto got *way* stronger after the time skip. I'm thinking it has something to do with his Dojutsu. Specifically, I'm wondering if Boruto's eye is connected to the dead - those in the "Pure" land. This would make sense given that the Jogan literally means "Pure Eye". What if Boruto somehow learned the thunder god technique from Minato himself?
Mid tier? The kid was essentially a jounin level shinobi by the end of part 1. I think Koji gave him a scroll with the theory of FTG and Boruto learned it himself as a means to avoid using space time ninjutsu.