When fixing the corners like at 8:33, I noticed there's ngons next to corners. Are they okay in this situation? Any tips on how to fix it if it should be quads? :)
Good catch! I think I just didn't notice them there. They don't cause any problems here since it's a flat surface, so I wouldn't worry. If you do want to make them quads just for fun, you can pretty easily connect things like so all the way around and delete the edge between the two triangles in the corner:
