I think I've figured it out. The piece seems to be in Am key, chord progression is: F7, E7, Am, C7 (at least I think it's C7, although he's not playing the E) and the super important scale he's using to give the "contemporary melancholic" feeling to the melody is: A C D D# E G G# A. Here's where my music theory stops, I have no idea if that scale has a name or not. But it sounds beautiful! 😃👍 英語で和音や音階を説明させていただきました。さすがエモい感じが良い!