who is it or who is this

What is the difference between "who is this?" and "who is it?"

is "who is this" more specific to asking "who's calling" on the phone? What about someone knocking at the door?

when can I use " who is it"?

You are correct.  If you answer the phone, you ask "Who is this?"  However, if you then call out to your wife that someone wants to talk to her on the phone, she would ask you "Who is it?"

In most circumstances, you could say that 'this' is used as a way of 'pointing', either with your finger or just verbally. 'It' is used when there is no need to point because it has already been established what thing or person is being talked about.

And you are correct about someone knocking on the door.  We say "Who is it?"