Why is it "ez az" instead of just "ez"?
To say "this/that [noun]" you have to add the definite article in Hungarian.
Why not "Ez kocsi szép"?
Because "this car" is ez a kocsi -- you need the definite article in there in Hungarian.
Your sentence means something like "This is a car is beautiful", I think.
Why not "ez az szép autó" ?
That would be "This is the beautiful car"
since szép doesnt start with a vowel, the definite article will be "a" but if it starts with a vowel, it's "az", because pronounce two vowels next to eachother creates an unpleasant sound