On the other hand, I don't really understand why things that look like questions but aren't, like "how do you do" or "what's up?", are used as greetings.
I understand that they are, and it's been years since I last made a fool of myself by misunderstanding and giving an actual answer instead of returning the greeting as expected by common social protocol. But I don't get it. I suppose it feels like a second language that's still a second language even though you've been speaking it fluently for years.
I'm not alone in this. Because of the literal-mindedness that often comes with autism, Autistic people can have trouble telling rhetorical questions, and greetings that look like questions but aren't, from things they're actually supposed to answer.
One way around this is the way I've gone - a lifetime's worth of learning by muddling though social interactions to the point where I've figured out (touch wood) what is a real question and what isn't.
The other way would be much easier for all concerned: don't ask unless you want to know.