누가 is a shortened word of 누구가 and it is used when you ask some questions but usually we don't say '누구가' in this sentence. 누가 미국 사람이에요?(0) 누구가 미국 사람이에요?(x) 누가 전화했어요?(0) 누구가 전화했어요?(x) 밖에 누가 왔어요(0) 밖에 누구가 왔어요(x) 누가 내 과자를 먹었지?(0) 누구가 내 과자를 먹었지?(x)

i will tell u a grammar ^^ use 누가 when u want to ask about Subject ex: 누가 와요? < who’s come? use 누구 when u want to ask about Object ex: 누구를 좋아해요? < who’s u love? But there is Exception: use 누구 +세요 for polite way ..even if u ask about Subject ^^ ex : when someone Knock on the door u say " 누구 세요?" Hope that was helped ^^