(formal) hello >= hi > hey (casual)

Oliver said, "when you don't know what to say, the safest thing is to say "hello", then you can move on to "hi" and lastly when you are getting closer, you can start using "hey". So, remember this order: first hello, then hi, and lastly, hey.

https://steemit.com/oliversam/@wikitree-europe/koreans-nightmare-is-hello-considered-polite-and-hi-informal-oliver-sam-reveals-the-truth

https://www.youtube.com/watch?v=uaj1mK1sk8I