I would like to point out that in university-level English, the following are (or used to be) standard:
1. what type of car
2. what types of car
In other words, it is the word before the 'of' (type/kind/sort) that is singular or plural, whereas the word after the 'of' is singular regardless.
That used to be the standard, but nowadays you see plural after the 'of' quite frequently. In fact, the tendency is to use a double singular (kind of car) vs. a double plural (kinds of cars).
Do you see the cars or do you see the types/kinds or both? What I think is going on is that what types of cars is shorthand for what cars of what types, whereas what types of car would be asking for actual knowledge of the types, maybe "Milyen típusú autókat" (Can someone please correct me? Well, it did get 236 hits so maybe there's something to that.) and certainly above Duolingo's level.
When you're asking "Milyen autókat látsz?", your dialogue partner sees some cars and you want to know what they all have in common. Colour, propulsion method, size, manufacturer, something like that. How can those cars be grouped?
Maybe it also helps to know that the answer to the question milyen? is olyan, meaning "such" or "that kind".
And yes, "milyen típusú autó" would specifially ask about the "type" of car.
I don't think the sentence implies the cars have something in common - that circumstance (having something in common) only makes this question easier to answer. "Autókat" implies there are multiple cars and one asks to describe them. Of course you can cut the answer short by saying "Különbözőeket." but you could also say "Látok egy piros '57-es Cadillacet, egy albán rendszámú 120-as Škodát, egy roncs Mercedes A osztályt meg egy nem tudom, mit" :D