Plurals and possessives
It is generally pretty simple

car = singular
cars = plural
car's = possessive/belonging to the car

The headlights belonging to five cars are five car's headlights not five cars headlight's.

Plurals do not take an apostrophe. At all. Ever. No. Well they do in one case but otherwise not ever.

Capital letter
Capitalization is not some expression of terrible word inegalitarianism. They...