The huge apartments are kind of an acceptable break from reality for me. Yeah they occasionally have in-universe handwaves but the real reason is it’s a set and it would be hard to film in anything smaller, especially in a multi camera sitcom like Friends.

I didn’t watch enough Sex and the City to see if they had an explanation for everyone’s wealth, but it was pretty clear that all the characters were wealthy.

You have to consider they only have 2 or 3 walls, and usually no ceiling. Very rarely do they ever acknowledge the Fourth wall.