For the two Cash songs it sounds like he's playing the root note once, when he changes chords, then strumming the higher intervals of that chords to fill the voicing between each chord change.

Were there a bass player involved, those low bass notes would be his/her responsibility.

Cash is just breaking up his chords to accentuate both the bass line and the higher notes.

On the two Sarah Jarosz songs, it's the same thing with an emphasis on finger picking. She's picking the root notes with her thumb and then catching the higher notes with her first, second and third fingers.

You can see it in her music video for "Run Away:" https://www.youtube.com/watch?v=lKBN5ewEXdc