Experience
At the end of each race, you will earn experience based on your performance. If you have extra money, you can also level up outside of races through training.
Your driver gains experience based on the number of successful overtakes you pull off. A higher level driver will allow access to better upgrades.
Your mechanic gains experience based on how little damage your car took. A higher level mechanic will restore the your Service gauge faster
Your sponsor gains experience on whether or not you met the position criteria, which was indicated at the start of the race. The higher level your sponsor is, the more bonus money earned after finishing a race.