In this context, no difference. But kullanmak can also mean "to use", and sürmek can also mean "to last" in other contexts.
Just think of kullanmak as "use" because what do you do when you are using a car? You're basically driving it. Turkish just tends to be a bit casual about these definitions.
When you look at any language from another language, the same tends to be true.
how can i say "who uses the car?" ? Meaning, how can i say it in present simple?
The word "this" (bu) is not included in this sentence. :) That is why.
Is there a particular reason why we do not need a -mI particle here? Or is it optional?
True! Maybe there is a little exception: you can use it with question words to question the question itself :)
-- Ne istiyorsun? -> What do you want?
++ Ne mi istiyorum? -> What do I want? (Are you really asking this?)
So "Arabayı kim mi kullanıyor?" would mean "Are you asking/wondering who is driving the car?"
That is a grammatically correct English sentence, but Duo in general wants you to use present simple tense to translate Turkish aorist, and present continuous to translate Turkish present continuous.
This one uses present continuous -ıyor and so it expects present continuous "is ...ing" in the translation.