It's in the key of A Minor but he threw a G# in there. Yet, it sounds like it's still in key. I've been trying to wrap my head around this for months and I still can't understand it. It's similar to borrowing chords but this is only throwing an extra note in. If he were to play the correct 7th for the G it would be a G7, which sounds very tense and needs the resolution of a C chord.
What I want to know is how do ppl decide which chord to alter even if it means using a note that is not in the scale of their song? Everytime I try to do something like this, it sounds very out of key and doesnt work like theirs.