Buonanotte it's always used as "goodnight", meaning that you only get to use it when you're literally in a situation where someone is about to go to sleep.
["Buonanotte amico, riposa un po', ne hai bisogno" - "Goodnight friend, rest for a little bit, you need it" ]
It can also be used ironically sometimes, but this is more of an idiomatic meaning: for example when you're explaining something and the listener misses the point completely, sometimes you'll hear the person that just wasted his time and patience yell "se, buonanotte" in frustration. This can be considered a little rude though.
Buonasera it's simply a greeting you use at night time. If you meet someone you know (or even if you don't actually, it's not that uncommon to greet people you don't know personally but have seen or spoken to a few times) 7PM; you greet them with "buonasera"; it's way more polite and appropriate than just saying a generic "salve" or even worse "ciao".
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