Half joking, but Berlin techno :)

You can have a whole set developing motifs, then multiple minutes of a single note being played in the middle, and somehow that sounds awesome (if you’re into it)

Adam Neely has an interesting video about one-note muisc, with some good (and bad) examples: https://www.youtube.com/watch?v=eSuK_5zW2iM

I can only think of examples which as you describe have accompaniment moving underneath. Neil Young's solo in Cinnamon Girl is one example. Another is Bob Dylan's Like a Rolling Stone in which he sings one note as the chords run up C Dm Em F and only breaks on the G.

A lot of modern metal is mostly one note played rhythmically. And it is can sound awesome, however timbre is obviously part of this awesomeness.

Chariots of Fire is pretty dope.

I'm pretty sure you've heard plenty of ambient music and just never thought about it. Timbre and rhythm have been the focus of modern music for at least 40 years.

I'm finding this difficult to imagine

Maybe go on Youtube and listen to music that you might not be familiar with?

Cage, Schaffer, Stockhousen, Boulez are some composers in the tradition of European classical music.

If you go outside that tradition, there’s even more. Endlessly more.