The A section is pretty straightforward, largely arpeggios and some conventional scalar passing tones. There’s little dissonance beyond major sevenths. However, although the phrases are arpeggiated, the overall line starts out chromatic: landing on the major seventh, going down a half step to the third of the minor v, then down an half step to the third of the IV. It takes a whole step down for a brief suspension on the fifth, and then it goes to a 3-note motif in the major scale: 3-4-5 on the I, 3-4-5 on the vi, 3-4-5 on the ii (with an octave jump and implying the 4) and 3-4-5 on the V to end on the 3 of I.
All of that is rendered more interesting by chord substitutions and turnarounds, but the melody on its own stays pretty tightly in the key.
The bridge, on the other hand, creates tension and keeps it building on a single chord for two bars, resolves for two, builds tension on a single chord for two, and spends two bars in a normal turnaround. I do a lot of substitutions, but they don’t change the tonality: instead of Bbm – Eb7b9, I do an edim-Bbm, and for the amin7sus I do some diminished chords that all spell out the same. The A section comes back in predictably.
Overall, it’s probably the use of arpeggios with mid-stream octave changes that obscure the simplicity of the melodic idea without losing its comfortable familiar consonance. It wouldn’t work without the tension, and cramming all of that into a few bars in the B section is perhaps overcompensating. I don’t really see a lyrical reason for the contrast, but it makes a nice overall arc to the song.