What Mr. Popo taught Goku in the beginning was how to think quickly while remaining calm simultaneously. Technically .... Goku has been working toward UI since he was a kid without even realizing it. In the episode where kid Goku fights a fake version of himself, he stands still, closes his eyes, and takes a deep breath right before his opponent strikes, dodging just like he does with UI. Later in Super, Goku spends more time meditating and calming his mind than doing intensive physical training like Vegeta. Chi-Chi even complains that he is acting strange, and Goku hints there is a specific reason for his new training. Whis clearly knows what is happening but hides it from Vegeta, and even Gohan suggests that Goku has discovered a new method. A big clue appears when Bulma tries to punch Goku and he dodges it like UI (pretty weird for in character Goku), which Goten says is very unusual for him.
The entire anime details Gokus path to this godly power. He uses a fraction of it for a second against Beerus which Beerus notices, since he was ready to kill Goku with said blow ... and again to fight Hits time-skip. This also connects back to the ginyu crew saga. When weirdo Ginyu took Gokus body, it was weak because it lacked Gokus mind. But by the time Zamasu steals his body, Goku has trained so much that his body has muscle memory for god ki. Goku Black inherits techniques like IT and the kameha immediately because the body is so fine tuned for battle. Gokus instinct and battle lust were so strong that they even changed Zamasus personality, making him stop caring about immortality. The whole series was setting up the idea that Gokus body was learning to fight on autopilot ... since episode 5 or some shit.
UI is just peak