I learned about chord borrowing a very long time ago from a book by Don Latarski and it’s very useful when writing tunes, it adds color…~TS
A borrowed chord (also called mode mixture and modal interchange) is a chord borrowed from the parallel key (minor or major scale with the same tonic). Borrowed chords are typically used as “color chords”, providing harmonic variety through contrasting scale forms, which are major scales and the three forms of minor scales. Similarly, chords may be borrowed from the parallel modes, the various modes beginning on the same tonic as a scale, for example Dorian with D major.
In the key of C Major, the regular diatonic chords are built as triads (or seventh chords) on the roots of each of the seven notes of the C Major scale (the notes, C, D, E, F, G, A, and B). The chords (triads in this example) built on these scale degrees (i.e., notes) would be C Major, d minor, e minor, F Major, G7, a minor and b diminished. In the key of c minor, the regular diatonic chords would be c minor, d half-diminished, Eb Major, f minor, g minor (or G7, as the leading tone is often sharpened), Ab Major, Bb Major. Thus, a song in C Major could “borrow” chords from c minor. For example, a simple song chord progression in C Major, such as I-IV-V (C Major-F Major-G Major) could have chords from the tonic minor added in. Thus a new chord progression could add bVI (Ab Major) and bVII (Bb Major), thus giving us I-bVI-IV-bVII-V or C Major-Ab Major-F Major-Bb Major-G Major.