> Why update a static screen at 120 fps?

Good thing it doesn't do that then, variable refresh rate displays that go down to 1 Hz are fairly standard now on phones as well as other displays.

Even before that, mobile UI frameworks are retained mode GUIs, not immediate. They aren't drawing to a blank framebuffer 120 times a second if they don't have to. Redraws only happen when something changes (e.g. "Dirty" rects).

Oh even immediate UI framework don't paint non-stop. If the UI has not been interacted with, or if there are no animations/gifs, it has no blimey reason to repaint, and it will not. It will repaint the whole screen, of course, but that's already a win.

They don't. GPU rendering only happens when something changes. Even composition only happens when something changes thanks to panel self refresh (this is independent of the more recent VRR that also lowers refresh rate when idle, this is a relatively small savings compared to the other two)