What made it click for me analyzing music, in particular rock songs like 'Gloria.' That song very strongly identifies E major as the tonic, but the D and A cho

What made it click for me analyzing music, in particular rock songs like 'Gloria.' That song very strongly identifies E major as the tonic, but the D and A chords are not in E, they are diatonic to A major. To say it is in A major would mean the song's tonic would be A, but since it is E major it is more correct to say the song is in E Mixolydian. Adam Neely recently did a great analysis of 'Hey Joe' that goes pretty deep into this stuff https://youtu.be/DVvmALPu5TU https://youtu.be/DVvmALPu5TU