Adding the “what’s up” is important though, or something to the same effect (“how are you”, …), because it helps support the conversation and keep it flowing.
Just mirroring the “Hey” can feel terse and unwelcoming, like you’re not interested in talking because you’re not providing the other person a “conversational path” they can follow up on.
That sounds like a pretty interesting cultural difference. Around here it would be pretty normal to just reply with a “hey” back, as it’s clear the other person has something to say because they’ve sought contact with you in the first place, so it’s on them to keep the conversation flowing. We also often don’t say names when we interact with each other, so instead it could go something like:
“Hey!”
“Oh hey”
“So, what’s up?”
That sounds like a pretty interesting cultural difference. Around here it would be pretty normal to just reply with a “hey” back, as it’s clear the other person has something to say because they’ve sought contact with you in the first place, so it’s on them to keep the conversation flowing. We also often don’t say names when we interact with each other, so instead it could go something like:
“Hey!”
“Oh hey”
“So, what’s up?”