Let's look at the options when creating these 7th chords from a C root note. For Cmaj7 the notes would be - C (root), E (3rd), G (5th) and B (7th). For Cm7 the notes are C (root), Eb (b3rd), G ...

To begin with, we just insert a new chord on the downbeat of each bar. Notice how Am9 and Cmaj7 are basically the same chord (C-E-G-B), but playing these four notes over a C bass rather than A ...