So I'm currently learning about using resolution in guitar chord progressions, and so far I have learned about tension chords which is usually, but not limited to, the fifth chord in the scale that helps you resolve back to your tonic. I'm also learning about backdoor cadences which can also help you resolve back to your home or I chord. As a little practice for myself I'm going through random scales and trying to find what the ♭VII chord would be in them. For D major I learned it would be C because when you flat a note in the scale that note goes down a half step in the scale, and it so happens the 7th degree in D major is C♯. So would the ♭VII chord in A major be G?
Yep. in general the bVII chord will be the major chord a whole step below the tonic. Likewise bVI will be the major chord a whole step below that.
You might find this useful: https://www.secretsofsongwriting.com/2011/05/06/using-a-flat-vii-chord-in-your-songs-progressions/
Just to add to Don's answer. Take care how you name those ♭VII chords. They will always bear the penultimate letter name of the scale.
D>C and A>G are simple. F♯ goes to E, B♭ goes to A♭ (not G♯), A♭ goes to G♭ (not F♯). Although they'll sound the same (they're enharmonic in 12tet), they might as well get the right name!