Turn 10 Studios did something different with Forza Motorsport 8 by making you level up your cars. In prior games, you could just spend credits to upgrade cars, which was a straightforward and meaningless method for growing your garage. However, you have to level your car by using it to unlock upgrades. Furthermore, upgrades have their own currency, separate from the credits you use to purchase new vehicles.
https://screenrant.com/forza-motorsport-8-level-up-cars-fast/