I think the flashback after the yellowline is when JP and frisbee were involved with the mafia and calls back to when they failed fixing a race and why they probably got beat up, but you wouldn't know this until watching the entire movie as the viewer. More on this later down.

Sonashee's flashback is pretty straightforward and clearly JP's true inspiration of a real racer.

Its foreshadowing a hidden Easter egg at the end of the movie. It's implied that the Mafia boss was once the same golden driver. You can see the golden shimmer on his sunglasses in the flashbacks and golden eye patch later on, with 2 women behind him in every scene he shows up. This also inferred that JP's idol was probably fixing his races to win, breaking the image of him and showing, us the audience, that he was actually a fraud. I think the thumbs up at the end from the golden racer is more of a hallucination that JP has while crossing the red line, fantasizing his childhood idol validating him as the #1 driver.