You have in front of you a staircase with twelve steps...
You have to get to the next floor, only stepping on 7 steps, but you HAVE to step on the first step...
You have several ways of getting to the next floor, right??
Well, every combination is a scale.... (7 steps= seven notes) Since they all have the same starting point, the are related at least through their root note (the first step)...
Changing the way you 'll go to the top (changing your set of steps) will change the sound of your progression or solo... This is pitch axis theory... You change your set of notes, but keeping the root the same. By the way, not all changes sound good!!! We actually use the ionian/myxolydian everyday in blues music (from a C CHORD TO A C7), and the ionian/aeolian (major/minor) in pop songs...
All this theory is actually useless if you don't use your ears to write music!!!