18.1Start with the major scale

In C:

C D E F G A B

Scale degrees:

1 2 3 4 5 6 7

You already know this.

Over a diatonic ii–V–I:

Dm7 → G7 → Cmaj7

all three chords can be played using notes from C major.

But the important targets change.

Over Dm7, emphasize:

D F A C

Over G7:

G B D F

Over Cmaj7:

C E G B

So the same scale becomes three different sounds because the harmony changes beneath it.