Without being learned about theory, I suggest that folk songs (or European and American ones, anyway) don't modulate. They stay in one key, so the question is, when to change chords. Joe Offer had a very good answer to that one on a recent thread, maybe someone remembers which.

"Chord progressions" are useful in understanding jazz. Blues has 3 or 4 simple chord progressions; jazz has more complicated. But in other Western music, the organizing principle is tonality, and the chords (if any) derive from the tune, not vice versa.