Hi, Carol. Yes, good, but big question.
When you hear close harmonies, such as CSNY's, they are usually adding extensions to the chord. So you may be hearing one of the voices singing the major7 or 7, or 6 or 4 of the chord. Remember that any note coming from any instrument or voice in the overall sound becomes a chord tone. The trick with it always is to extend the chord in a pleasing, appropriate way.
The F#m I inserted in Silent Night: the simple change would have been back to the I chord (A in this case) from the E7 (the V chord)... I went to the relative minor instead, the F#. It's as close as you can get to the I chord, but you can hear that it didn't resolve anything, it demanded I go back and play the IV to V again so that I could finally get back 'home' to the I chord. I'm not exactly sure what inspired me to arrange it that way, but musically, it's all well within the key and is not overly complicated.
The A7/C# in the Waltz came about because I wanted the bass line to climb back up to the D. It moves up from A to B to C# to D. Chord wise There are several possibilities to choose from:
A Bm7 C#mb5 D
A Em F#m D
A G F#m D
A E7 A D
A B7 A D and more...
but to keep it nice and clean and strong and conventional sounding, I chose to stick to the I-IV-V chords: A G A7 D and put those bass notes underneath: A G/B A7/C# D.
Easy!
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