"Hej!" is the word Poles use to greet somebody. It's relatively new word and the more official word is "Cześć!". "Hey!" is used rather between close friends

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And "Jesteś." means "You are." but no one rather speaks like this. If you want to tell: "Oh, you're here!" you should say: "O! Jesteś tu/tutaj."
The question written "Jesteś?" means "Are you there?"
I hope this helps

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