grammatical number - Cars manufacturers or car manufacturers?

While referring to an attribute of a group of objects, should the attribute alone be pluralized or should the objects be pluralized as well? Is there a rule that can be used to identify which word(s) have to be pluralized?

Which is the correct phrase that has to be used in the following scenarios?

Eg 1: If I have a list of companies that make cars, should it say "These are the car manufacturers" or "There are the cars manufacturers"?

Eg 2: Student names v/s students names. (list of students)

Eg 3: Student count v/s students count. (a single number with the number of students in the list)