The pre-chorus is in, well, we can say 3 flats. First there’s an Ab Eb/G resolving to Bb (which sounds like the tonic), then a little Abmaj7/C Bb/C Cm build up that ends with Abmaj7 Ebmaj7/G Ab (the key sounds more like Eb here). Then things get weird.

Ab is followed by Ab7/Gb, but while it sounds like the bass is heading down toward a Db/F chord, at 0:42 the bass jumps back up into into this striking Ab13#11 with no 3rd.

The melody is also great here; just chord tones. It resolves to G7sus G7 Cm.

This Ab13 is basically a tritone sub for D7, but the 9th and #11 decorations also evoke the sound of a bVII dom7 “backdoor” chord in Bb. The key briefly sounded like Bb in the pre-chorus, so I think that resemblance isn’t accidental, but anyway… This was a tough one to figure out by ear. I could hear the embedded aug triad, but the recording is right in the cracks making playing along basically impossible. I had to retune an acoustic guitar with ancient, very dead strings.