Modes In Series

View the full lesson at Modes In Series | JustinGuitar

At the end when you did G7 to Bb7, why was mixolydian chosen for both? I suppose you could’ve use Ionian or Lydian also right?

Hello @kmason14 and welcome to the community.

The lesson is about modes in series so all derives from the notion of a parent major scale and its diatonic chords. From there, there is only one chord that becomes a dominant 7 chord which is the V chord built on the 5th scale degree. The 5th mode is mixolydian. Hence, when playing over a simple non-diatonic 2-chord vamp such as this, mixolydian is the best choice.
If there was a chord progression that was very diatonic such as C, Am, F, Dm, C, Em, F, G7 then the fact of it having a G7 does not automatically flag up the need to use mixolydian over that dominant 7 chord. The progression is very much Ionian.
I hope that helps.