That's... really not how this works. As a person designing lithium management circuits, you typically have a very small delta-V for top-off charging. We're talking ~0.1V. Once full, the charge circuit disconnects. If the battery is under load or self-discharges below your dV, charging starts back up usually at a very low current. Top-off is also totally optional, it's just a common feature.

Additionally, only the cheapest of cheap garbage phones would behave the way you imply. Charge controllers are readily available from TI et al with battery bypass. When the battery is full, the system runs on external power exclusively, except for the top-off charge.

It's really a pretty standard feature. You can get controllers with and without this, but you'd have to be either extremely cheap or extremely dumb to not use a controller with battery bypass in an application like a phone.

Based on my own testing, my pixel 8 will run on USB. No current fluctuations that would indicate battery cycling, just a constant draw. My previous Samsung also behaves this way.

You're right that cycling the battery this way is bad for them, that's why competent engineers don't.