Does every prebuilt car have the max amount of 20 bricks on it? If not then would it be beneficial to build a car for each world before the run (with each one having 20 bricks for max health) and then choose it during the run?

The number of bricks does not affect the car in any way as far as I can tell, the acceleration and speed don't seem to change at all between different cars, and cars with more bricks on them lose more bricks when they get damaged.

You could however choose the chassis that you prefer for each world as that affects the collision of the car with the terrain which can make some parts easier.

correct me if im wrong, but it would be better to have max bricks since the more bricks, the more boost you get from getting damaged

the implication would be, I believe, that you don't gain brick boost proportional to the number of bricks that come off your car, but rather proportional to the health you lose (and all cars have the same)