Well I got to the first of your linked videos before bedtime and wow this was REALLY interesting. More tomorrow, but then do I understand for BIAB RTs by professional musicians, they would have the musicians play every chord imaginable within reason so that a chord could be selected by the software to match what the user inputs for his/her song? The studio sessions must be pretty interesting then with the players playing an unusual chord chart covering all the needed chords. How granualar do they get? Do the artists play even the very unusual chords with odd extensions or just the basics and then the software will make the closest substitution like your first video mentioned when discussing User Tracks?

How does this work for non chordal instruments like solo single line instruments? Does the artist record a whole group of single notes that can be used for every eventuality?

I think knowing this is giving me an even greater appreciation for what is going on in these backing tracks. How totally interesting.