They are various forms of greeting that quite often don't actually mean the specific wording. It's kind of a cultural thing, for example where I am this could be an opening conversation: Person 1: What's up? Person 2: Just relaxing at home, you? Person 1: Yeah not much. The "What's up" means both a kind of "hello", "how are you", and "what are you doing" in one combined greeting. It is the same with "what's going on". It is basically an open door for you to say if there is anything in particular happening with you. For example: P1: What's up? P2: Not much, but my mums a bit sick at the moment. P1: What's going on? P2: Ah just playing some pool, going over to Jack's later on. think of it as a general greeting, the reason people respond with "not much" or etc, is because that's basically the usual greeting response. If something actually is going on (for example mentioned above) you would respond like that. It's kind of a cultural thing in a way to respond as though there isn't much going on, unless there is something paticuarly big happening at that moment or you want to talk about something further.

this is a common question- The "what" in "what is up" refers to something outside of you, the situation, the event, the actions occurring near you. Whereas, "what are you doing" is emphasizing what you yourself are doing. what's up = what's going on = what thing is up in the air at the moment = what's the current situation like for you? what are you doing = what is the thing you are doing right now normal answers to "what's up": - nothing much (there is nothing much going on) - not a lot - you know, the usual (i.e., the usual things that happen every day) - [I'm] just at work (what I'm currently doing now) - working - on the phone this page is interesting: https://en.wiktionary.org/wiki/what%27s_up "Made popular by the cartoon character Bugs Bunny who used it as early as 1940 as part of his catch phrase “What’s up Doc?” (where each word was pronounced distinctly)." Here's the famous cartoon : https://www.youtube.com/watch?v=UeVtZjGII-I

I am not English native speaker. But I know that 'what are you doing' = apa yang sedang kamu lakukan? . What u meant must be "How are you doing?". So 'what's up', 'what's going on', 'how are you doing' = how are you (apa kabar) and the answer 'nothing much' = ya biasa aja.. When meeting with my native speaker friends, I usually answer with this words, It's no problem for them, but please correct me if I am wrong: Native speaker: Hey what's up! Me : Hey what's up too! or.. Hey I am fine Native speaker: Hey..what's going on? Me : Hey..nothing's going on or.. Hey..Everything is fine. Ya..begitulah :)