Learn Finnish: Eri autoissa on eri vaihteisto. - Different cars have different gearboxes.

While muu means "other" (as in "the rest" or "something else") and toinen means "another" or "the other", eri specifically means "different" or "various". Here, it emphasizes that the cars are of distinct types, not just "other cars" besides the ones we were already talking about.

Why doesn't eri have the inessive ending -ssa to match autoissa?

The word eri is an indeclinable adjective, meaning its form never changes to match the case or number of the noun it modifies. Even though autoissa is in the plural inessive case (in the cars), eri stays exactly as it is.

English uses the plural "gearboxes", but the Finnish sentence says eri vaihteisto (singular). Why is that?

In Finnish, when a plural group of possessors each has one of an item, the possessed item is usually kept in the singular. Since each car has one gearbox, vaihteisto remains singular. Using the plural vaihteistot would imply that each individual car has multiple gearboxes.

Could I use erilainen instead of eri here?

Yes, but the grammar would change. Erilainen also means "different", but unlike eri, it inflects to match its noun. If you used it here, it would become erilaisissa autoissa on erilainen vaihteisto. Using the short, unchanging eri is very common and natural in this context.