안녕 مقابل 안녕하세요

안녕하세요 is the default with anyone you do not drink with yet: staff, landlords, your boss, a stranger on the phone.

안녕 is for kids, close friends, and people who already switched to 반말. Using it on a clerk is not 'casual cool'. It is rank-blind.

안녕히 가세요 (they stay, you leave) vs 안녕히 계세요 (you stay, they leave). Get this wrong and you still survive. Get 안녕 wrong and the air changes.