Some popular ways people say "Hi" in American English:

Hey – Very casual and friendly.
Hello – A bit more formal but still common.
What's up? – A casual way to ask how someone is doing.
Howdy – Friendly and a bit Southern in style.
Hiya – Light and upbeat, often used informally.
How's it going? – Another friendly way to ask how someone is.
Yo – Very informal and often used among friends.
Sup? – A shortened version of "What's up?"
Good to see you – Often used when greeting someone you haven't seen in a while.

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