in regards to 201 module 1 lesson 2, with modal borrowing you talk about substituting “vi” with “flat VI”. For C major I understand this as C-minor A is flattened(VI=Am->bVI=Ab). Does this rule for “VI” to “bVI” apply to all keys? Is there a “rule” to follow with modal borrowing like “flat X” or do one need to always go with “go to same key with minor, and substitute from there (like vi to VI, iv to IV, ii to iidim)?
The best way to think of it is the root of the vi chord in major, moves downward by a half-step, and the chord is a major chord instead of a minor chord.
So C major has A minor as the vi chord, and A♭ major as the ♭VI chord.
Db major has a Bb minor as the vi chord, and a B𝄫 major as the ♭VI chord.
C# major on the other hand as an A# minor as the vi chord, and an A major as the ♮VI chord. In this case, we call it a ♮VI instead of a ♭VI.