What is in it
A minor triad is the root, a minor 3rd above it, and a perfect 5th above the root. From C that gives C, E flat, G. Built from thirds, it is a minor 3rd on the bottom (C to E flat) and a major 3rd on top (E flat to G), the exact reverse of the major triad's stack. The outer interval is still a perfect 5th, which is why minor and major are twins rather than strangers.
On the staff
The one note that changes
Everything that makes a chord minor lives in the 3rd. C major and C minor share the C and the G; only the E moves, down to E flat. Because the 5th is untouched, a minor chord is every bit as stable as a major one. It does not pull or strain. It simply sits in a darker place. That is the distinction to train: both chords are equally at rest, and the only question is whether the colour is bright or shadowed.
Where it sits in a key
Minor chords are not an exception to major harmony; they are woven through it. In a major key the triads on the 2nd, 3rd, and 6th degrees are minor (ii, iii, vi), and the vi in particular is the relative-minor chord that lets a bright key turn briefly inward. In a minor key, the home chord itself is minor (i), as is the chord on the 4th (iv). Half the chords in common use are minor.
How Aubel handles it
Minor starts unlocked alongside major, the two of them forming the opening pool. They are paired because the only thing separating them is the 3rd, and hearing that one note cold is the core of chord recognition. Each round plays the chord as a block on a random root. Clear the pair and the diminished triad unlocks next, which keeps minor's lowered 3rd and lowers the 5th as well.