There's 1000 potential levels to unlock stuff. Clothes, liveries, wheels, and discounts on the new cars. Appears you randomly unlock a car discount every 5 levels, other levels have set prizes. You get XP for doing street/pursuit races, hanging out in the car meet, doing time trials/race things in the car meet test track, buying the new clothes at the car meet, and buying upgrades for your car while at the car meet.
Since you get XP periodically for just hanging out at the car meet, you can basically AFK in the corner with your analog sticks rubberbanded and get XP that way but I'm not gonna bother.
Street races are basically just races, but have cars parked around and stuff, they have shortcuts, and for now at least you can only use the new "Tuner" cars in them . Sacrifice races to learn where the shortcuts are... some only help a little, but some help a lot, and many are kinda tricky (narrow or have objects to hit), but once you figure them out it will make a big difference. There's only like 6 of them so it doesn't take long to learn them.
Pursuit races are point to point races with few checkpoints, and police chasing you. They should be fun, but my experience is that the police and traffic are so laggy and wacky that it makes it feel more like a lottery. Maybe they work good on SX/PS5 but on XB1 they are awful.
You can buy an "auto shop" now... but unfortunately it is accompanied by the worst NPCs yet, Moodyman and his girlfriend. They are painful to listen to, but the garage is kinda cool... Definite "car guy" customization options, 10 new spots for cars, can do mini-heist style missions from them that have 2 setups and a finale, customize cars as "legit" work where you don't have to fight NPCs and stuff, and find cars to export. None of it will make you more than Cayo Perico runs, but it's a nice change of pace, and it appears it can all be done invite-only sessions and solo.
The new cars fall into a special "Tuner" category which allows muscle cars to race sports cars and so on. For those looking for the fastest of the new cars to use for some races, the Calico GTF seems to the be the one. It doesn't handle amazing but has monster acceleration. I've just been using a stock one and have won 20 something races today/tonight.
Can also remove radio stations from the radio wheel now, which is amazing.