When somebody is knocking on the door, you respond with: "Who is it?". Why 'it', while you know it's a human being who is knocking on the door? Can you respond with: "Who is he/she?" Or even "Who are you?" as the Chinese do. Is it just a language habit or is there some logics behind it?

And when you pick up the phone and someone asks for you, you are supposed to responed with: "This is he." Why not "This is I."?
Normally we say "May I know who is calling?".
Rishonly Normally we say "May I know who is calling?".
This wouldn't really be an appropriate response to a knock at the door.

You can say "may I ask who's calling" when you answer the phone and don't know who it is.

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No logic that I know of-- it's a social formula.

Who are you? is odd and rude; Who is s/he? is odder. Who's there? is a common alternative.

This is s/he in response to your name given is grammatical and polite: this is the person you are seeking.
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 MrPedantic's reply was promoted to an answer.