How does Roygbiv function, compositionally?

I don't have the musical background to describe what I'm asking any better than that lol.

When I think about BoC, one of the first things that comes to mind is their very particular taste in chord progressions. They tend to be kind of airy, suspended, and ambiguous. But as far as I can tell, Roygbiv doesn't really have a dedicated "chord" instrument. The progression seems to implied via the intersection of monophonic synth parts.

Maybe the bass is playing a chord, because it sounds like a two oscillator type of deal where one osc is playing the root and the other osc is tuned up a fifth. I guess that could technically be a "chord progression" since the same bass line plays the same part throughout the whole song. But I don't hear anything else being obviously chorded. The two (I think) monophonic synths sort of dance around the bass line which results in incidental harmony before the little honky tonk piano riff / snippet resolves the progression.

How does one write music like this? It sort of reminds me functionally of old video game music because the composers for those old systems (NES, Genesis) were often limited polyphonically.

What I usually do first is write a dedicated chord progression on piano or synth and then fill in the gaps from there. Any resources where I could learn more about near-total monophonic arrangement?