Hi Rob,
I'm not sure if this will answer some of you Questions ?

I'm basicaly a blues player:
I know that I could play a blues using three chords in Minors or Majors they both work.

If i started in A the other two chords would be D and E

A chord is made up of the notes 1,3, 5 of A major scale
D chord is made up of the notes 1,3, 5 of D major scale
E chord is made up of the notes 1,3, 5 of E major scale

A minor chord is made up of 1, flat 3rd, 5 of A major scale
D minor chord is made up of 1, flat 3rd, 5 of D major scale
ETC...

When soloing I would use A minor pentatonic scale: A , C, D , E, G.
Or A minor blues scale: A, C, D , D#, E , G.

Theses two scales would harmonise over all three chords, major or minor.

Or I could play D minor blues scale over the D or D minor

Or I could play E minor blues scale over the E or E minor
I don't fully understand why ?