What do the manufacturers have to do with the Sprint Cup Series cars?

Answer 1

The most important thing the manufacturers do is Homologation. It's a long word that means "make a car for the general public that is the same model as your race car." The tradition of NASCAR is to have people racing the same cars their fans drive, so if Ford wants to race the Fusion this year, they have to sell Fusions to the general public. If the automaker discontinues the car they race, they've got three years to create a new NASCAR version out of a car they still sell.

They also make the engine blocks, at least one body panel - the trunk lid is stock - and provide some of the parts used in the car.