Well, that's the thing. A scale is always a consistent set of intervals -- Ionian is when you start from the first and go all the way to the eighth (or one again). When you go from second to second (or ninth), it's Dorian.
So D Dorian is technically C major (or Ionian). But couldn't you also say that C Ionian is technically just D Dorian?
3rd to 3rd = Phrygian, 4th to 4th = Lydian, 5th to 5th = Mixolydian, 6th to 6th = Aeolian (minor), 7th to 7th = Locrian
They're all intertwined due to the fact that D Dorian basically means 'Take D major and flatten the 3rd and 7th'. And when you do that with D, it's basically like playing D to D with no sharps/flats (or in C major). Music theory is a concept that's dodgy, but once it's grasped it's almost permanent.
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