Translation:Who is standing next to that big yellow car?
Why is the answer insisting on "that big yellow car" rather than "the big yellow car"? There's no "az a sárga autó"
All postpositions do this - if there's a demonstrative az or ez in front of the noun, then it has to receive the postposition as well. And it will lose the z if the postposition begins with a consonant.
a piros alma (the red apple)
a piros alma mellett (beside the red apple)
az a piros alma (that red apple)
a mellett a piros alma mellett (beside that red apple)
ez előtt a piros alma előtt (in front of this red apple)
e mögött a piros alma mögött (behind this red apple) and so on.
It comes from the a mellett in the sentence. The a here is a short form of the demonstrative adjective az.
Ki áll a nagy sárga autó mellett? - Who is standing next to the big yellow car?
Ki áll a mellett a nagy sárga autó mellett? - Who is standing next to that big yellow car?
For me this sentence was incomprehensible, the vocal part of "a nagy sárga" Does with pratice this get better?
Sure, with practice you get used to it. The speaker in these recordings is actually very good and accurate, and you'll get the hang of it with repeated listening.
Why is mellett in the sentence twice: Ki áll a mellett a nagy sárga autó mellett. I've never seen a sentence need to say beside twice like this, unless ur talking about being beside one object that is itself beside another, which isn't the cae here.