It updates 6 cars for every "racing day". i.e. For every license test you move fwd/back, it +1 day.
For every A/B spec you race (must go until load race track), +1 day.
It works for seasonal also.

Fastest way to grind is to load license test #1, go next next next next until #5, exit and check your UCD.

Replay won't +1 race-day.

PP = performance point. This is to rate a car overall, as a car can have same PP, but car A (Veyron) maybe high HP, but heavy. Car B (Lotus Elise) lower HP, but very light. Therefore, both will have close fight on race track.
I just simply put Veyron and Elise as example, I don't think they have same PP.