Indeed, the D major triad is among the diatonic triads of A Dorian, that's true! However, I'm not aware of a borrowed chord technique that involves the relative minor in this specific way. In the context of C major, the D major chord typically functions as a secondary dominant (V7/V), rather than being directly related to the A Dorian mode. So while D major is diatonic in A Dorian, its use in C major is more about the dominant resolution than any connection to the relative minor.
Isn't the D major of C major just part of the relative minor in dorian mode? So C major key -> relative minor: a minor -> a dorian?
Indeed, the D major triad is among the diatonic triads of A Dorian, that's true! However, I'm not aware of a borrowed chord technique that involves the relative minor in this specific way. In the context of C major, the D major chord typically functions as a secondary dominant (V7/V), rather than being directly related to the A Dorian mode. So while D major is diatonic in A Dorian, its use in C major is more about the dominant resolution than any connection to the relative minor.