As a kid that used be the question that made me panic. Some teacher or parent would ask “What’s up?” In a casual manner and I would just freeze. To this day I have no idea what the proper response is. I assume it isn’t to start talking about orbits.
I look up, then answer the question. Usually by saying, “the ceiling”.
If it’s someone that gets “the ceiling” often, sometimes I’ll switch it up by describing the type of ceiling. Or a light fixture. The sky. Squint and say, “I think it’s a spider.”
When I was little I used to get confused and look up at the ceiling.
“Not much. You?” If you don’t have much going on.
“Not much, just finishing up this essay before I head to soccer.” If you are doing something that might interest the person asking, or to explain your presence somewhere you aren’t usually.
The person is just checking in with you. It’s basically “hello”
“Same old. The horror persists but so do I.”
“No, Signal”
That’s pretty good actually. I might use that at a cyber event.
“Livin’ the dream”
I usually say “not much”
It is a prompt meaning “if you have something you want to bring up, or that is important for me to know interacting with you, now is a good time. I have made a space in the conversation for exactly that and I am listening”.
So basically “nothing special” or “not much” is the correct response unless you actually are waiting to bring something up.
Continue with “…and you?” or “how about you?” is polite and invites them to also share anything relevant without worrying about “intruding” on your thoughts.
Caveat: Of course some people will say it as an empty greeting and not listen to a response. They are doing it wrong. If you respond with something and they are confused by that, it’s them being socially inept and not you.
Just reply back “hey”. No one is looking for a real reply.
The parent of one of my friends growing up wanted a actual reply. He asked me “What’s up?” every time we met. I would try to answer and he would try to get me to explain what I was up to. He had good intentions but it was a bit traumatic