"Who is it?"
I didn't understand why it doesn't need the वह or यह in that sentence, because in the sentence it has an it. Could you explain please?
This sentence may need some clarification. You don't use कौन है? when you see someone you don't recognise and ask someone else who that is. That would be 'वह/यह कौन है?'
'कौन है?' is used when you hear a knock at the door and want to know who is visiting (or in other similar situations). आप is implied so the question is really asking "Who are you?". The translation to 'Who is it' is idiomatic.