Apple put ProMotion in the built in display, so while it can ramp up to 120Hz, it'll idle at more like 24 Hz when showing static content. (the iPad Pro goes all the way down to 10Hz, but some early sources seem to say 24Hz for these MacBook Pros.) There may also be panel self refresh involved, in which case a static image won't even need that much. I bet the display coprocessors will expose the adaptive refresh functionality over the external display connectors as well.

It only takes a tiny animation (eg. a spinner, pulsing glowing background, animated clock, advert somewhere, etc), and suddenly the whole screen is back to 120 Hz refresh.

That's a useful tip for saving some battery actually, thanks.

Don't know much about the graphics on an M1. Does it not render to a framebuffer? Is that framebuffer spread over all 4 memory banks? Can't wait to read all about it.

The updates from the Asahi Linux team are fantastic for getting insights into the M1 architecture. They've not really dug deep into the GPU yet, but that's coming soon.

When nothing is changing, you do not have to touch the GPU. Yes, without Panel Self Refresh there would be this many bits going to the panel at that rate, but the display engine would keep resubmitting the same buffer. No need to rerender when there's no damage. (And when there is, you don't have to rerender the whole screen, only the combined damage of the previous and current frames.)

Don't Apple iPhones use an adaptive refresh rate nowadays?

Indeed ProMotion is coming to these new MacBooks too.