What is in it
A minor 7 is a minor triad (root, minor 3rd, perfect 5th) with a minor 7th added above. From C that is C, E flat, G, B flat. Notice that nothing in it is sharpened or stretched: minor 3rd, perfect 5th, minor 7th. That all-mild construction is exactly why the chord sounds so even.
On the staff
Dark, but at ease
A plain minor triad is dark and stable. Adding a minor 7th does not brighten it, but it does soften it. There is no tritone inside a minor 7 and no leaning toward a resolution, so where a dominant 7 sounds urgent, a minor 7 sounds settled and relaxed, dark without weight. It is the chord a soul or R&B groove can sit on for bars at a time without ever feeling like it needs to move. When it acts as the ii of a ii-V-i, jazz plays the Dorian mode over it; Mark Levine builds the minor 7th straight out of Dorian, the chord and the scale being the same seven notes read two ways.
Minor 7 against dominant 7
Both chords carry a minor 7th on top, so the 7th will not tell them apart. The difference is the triad underneath, and that comes down to the 3rd. The minor 7 has a minor 3rd, dark and shadowed; the dominant 7 has a major 3rd, bright. Listen through the chord to that third note: if it is low and heavy, you are on a minor 7; if it is bright, you are on a dominant 7.
How Aubel handles it
Minor 7 unlocks seventh. After two 7th chords built on major triads, the dominant and the major 7, this one moves onto a minor triad, so the cue shifts back to the 3rd. What you are listening for is a dark, stable chord made gentle by its 7th, with none of the dominant chord's pull. Half-diminished unlocks next, which takes a darker triad still.