13.31Where does the ♭9 come from?

For G7 resolving to C minor, G7♭9 is especially natural.

C harmonic minor:

C D E♭ F G A♭ B

Build dominant from G:

G B D F

and A♭ is the ♭9 above G.

So:

G7♭9 → Cm

is a classic minor-key dominant sound.