Yeah you can usually say "good, and you?", that woudnt really be weird. I do think it is worth mentioning "What's up?" can be used a few different ways. It can be a question, similar to asking "How are you?", that's when saying you're good in response would make the most sense, but I think that is probably one of the less common ways you will hear people use it.

Sometimes people say "What's up" as just as a casual greeting or way to say "hello". It might not be an actual question. You might see someone use it as a way to acknowledge someone they know, maybe in a public place or in passing, even if they don't have time to talk. So it can also be normal for someone to say "What's up" and then that's the end of the conversation.

I think another relatively common way it gets used is as an acknowledgement that someone is asking you a question, saying "What's up" in that way can be like saying "Hey, what did you need from me?". If you approach an employee at a store with a question, they might say "What's up" once you get their attention for example.

You'll obviously need to read the context to understand exactly what it means, I haven't thought about that a whole lot but I can see how it might get confusing if English wasn't your first language. If you just want a general rule though, I think it's probably best to just think of it as a casual greeting and that'll probably work 99% of the time, you're usually not obligated to tell someone what is actually "up" lol. Other languages do that too I think. It kind of reminds me of how "Nǐ chī fàn le ma" (literally "have you eaten?"), is used in Chinese as a kind of friendly greeting rather than a genuine question about food(but also kind of different, since you'd usually wanna respond saying something like "I have eaten, you?" in Chinese even though it's not really about food. I'm sure someone with a better understanding of Chinese could probably explain that better but that's probably for a different sub lol)