Modal mixture is for borrowing chords from other modes, so it looks at the secondary scale and add any chords from that scale that aren’t already present.https://en.wikipedia.org/wiki/Borrowed_chord
I refactored scalefinderOSC codebase to make this tool https://forum.renoise.com/t/new-tool-2-8-scalefinder-osc-0-1alpha/35076 but yeah scalefinder is the original. This one has some added functionality and better behavior but they are pretty similar.
song in the video is: