Being south korean myself, I need to say that "안녕하세요," is only when you say "hello" to elders, and people older than you. You say "안녕" to people who are younger or the same age.
I dont understand all of the different ways to say hi and hello? Does it have something to do with formality?
Yep. 안녕하세요 is formal and means both hello and bye depending on the context, is what I've gathered so far at least.
Yes! 안녕 is the most informal, 안녕하새요 is the standart and 안녕하십니까 is the most formal. The ending is different from the formal "ㅂ니다" because in the formal form, to ask a question you must change the ending to "ㅂ니까" accordingly