Q: Some people said somethinbg like secondary dominant but I have no clue what that is
A: i'm pretty sure it's not that, but even if it was, that wouldn't help you know a damn thing about why it sounds good. (or does it sound good? i forgot if you ever stated an initial opinion on that.....) The important thing to take with you for the rest of your life is that you now have some idea of what D and E chords can do in the key of A minor, and the feels it can bring, etc. Then with some simple extrapolations you could deduce that the same feels could also be produced in the key of Bbmin, with Eb and F chords. and so forth