Response to "What's up?" in various conversations [duplicate]

The usage of what's up is totally dependent on the context in which it is used.

As you've mentioned few scenarios above, what's up while chatting with a friend would usually be meant to greet a person in an informal way or rather to start off a conversation. So in such a context you could probably give a response telling your friend about what you are upto?

Whereas when the same is used in a formal way, as in while speaking to a boss/business partner, it can probably refer to any of these "What is going on?/ What's the latest news?/What is the progress of the work done so far?

All in all, it's just about the context, whether it is formal or informal. Based on these two contexts you can give a response accordingly.