"What do you do" usually means you are asking what someone does for a living. "What are you doing" is asking what someone is doing right at this moment. "What's up" is just a casual greeting, and doesn't always need an answer. Someone would usually say "good evening" when greeting someone, where as you normally say "good night" either before going to sleep or when saying good-bye to someone. I don't think anyone ever says "good night" as a greeting.

"what do you do?", <- what is your job? "what are you doing?" <- what is your activity in this very moment? "what's up?" <- how are you recently? also "good evening" <- when meet "good night" <- when go away

"What do you do?" is in the present simple tense. This sentence asks about someone's occupation. Ex. A: What do you do? B: I'm a doctor. "What are you doing?" is in the present continuous tense. It asks what someone is doing at that particular moment. Ex. A: What are you doing B: I'm making dinner. "What's up?" is very informal language. It can mean several things. If taken literally, 'the roof' or 'the sky' might be appropriate answers. It can also mean "What's wrong?" or 'What's the problem?". In American slang, it can mean the same as "What are you doing?", except it's used as a greeting similar to "How are you?".