The three people we usually talk about (and the same one, Shiranui Genma and Yanxi Tatami) were taught the art of Flying Raytheon by Watergate, because they are "four generations of Huo Ying guardians".
That is to say, at the end of the Third Endurance World War, when Watergate was the fourth generation of Huo Ying (enough time to teach Ninjutsu), they practiced the flying art of Raytheon.
At this point in time, Kakashi has been disheartened by the deaths of Dato and Nohara Rin, and he has no mind to climb. All he thinks about all day is how to choose a beautiful way to die.
Watergate doesn't want to teach Kakashi to improve his self-protection ability, but Kakashi wants to die and doesn't want to learn at all.
Four generations of Huo Ying and Namikaze Minato were the first Huo Ying with no family background in Konoha history, and Jiraiya, one of the three forbearers, was called "First Speed" and "Yellow Flash" and died in a terrorist attack planned by his former apprentice. Huo Ying's four generations have achieved excellent results since childhood, outstanding ability, calm personality and friendliness. When I was a teenager, I went deep into the border alone and rescued Jiu Sinai who was kidnapped by Yunyin. He is not only congenial to Jiusi,
Konoha has made great contributions, and can be described as Konoha's "young hero", which has certainly attracted the attention of Konoha's senior officials (it is an important national policy of Huo Ying's three generations to cultivate ninjas without family background to balance Konoha's family power). After officially becoming a ninja, Namikaze Minato became a famous teacher under Jiraiya, who also had no family background. It is impossible to speculate when the three generations of Huo Ying began to train Namikaze Minato as their successor. It was World War III that made Namikaze Minato famous and won the reputation of replacing Huo Ying. Although the Third Endurance World War has no positive systematic description, it can be seen in many places in people's memory (all the foreshadowing of the Fourth Endurance World War originated from the Third Endurance World War). As the shadow of the earth, Onogi found it difficult to cope with the battle against the bridge that the gods could not compete with. After the alliance between Konoha and Sha Ninja Village, it had to accept the Konoha armistice treaty, which played a final role in promoting the end of the Third Endurance World War. In the battle with Yunyin Village, four generations of Lei Ying and their younger brother Chilabi, who were still in stoppage time after Lei Ying, were impressed by an enemy and were ashamed (Namikaze Minato mentioned goodbye in the name of shadow when he left, so it can be seen that Namikaze Minato at that time may be in the training period of preparing to take over from Huo Ying). After the Third World War, Namikaze Minato's reputation increased greatly. It was after the Third Endurance World War that the opposition forces, led by Tuanzang, put pressure on the third generation of Huo Ying on the grounds of giving up claiming war reparations from the land, in an attempt to shake the Sarutobi Hiruzen regime at home and push the Third Endurance World War to continue.