Questions On Last Video Of Little Jazz Workshop (chords And Arpeggios)

Hi Ivan,
First and foremost, thank you so much for posting the videos and having such thorough material. I have a question regarding the notes on the last video of the Little Jazz Workshop (Chords and Arpeggios). You stated that "This is the Beebop scale...in conjunction with the mixolydian mode of that tonality." In this case it would be A mixolydian from the D major tonality. But how come later on you categorize the A mixolydian in the C Major tonality? My best guess is because A is the relative minor of C. But what if you decided to use a different modulation other than C major in addition to the D major? (then A mixolydian would not be able to fall under the C major tonality making the ability of placing the A mixolydian under the C major tonality an occurence of luck...hence if you use a different note (let's say E major) to modulate from D major, you wouldn't be able to use A mixolydian because A is not a relative minor of E major--in this case what other scales would you be able to use besides E major?) Thanks Ivan!
Sincerely,
Cliff