On my Sony Xperia I can force battery limit to 90% or 80%, but on my parent's Pixel 5a only the automatic/adaptive option was available? This seems like one of those things that's easy to offer, but Google hides it to be more slick and apple-like, while Samsung and Sony add the control in for power users / "value-add".
(There are other trade-offs between Google and Samsung, of course.)
The BMS, ie the hardware has to be setup for it, it isn't just a thing u can solve with a software limit.
I don't think you can. At least not out of the box. Pixels can do stupid adaptive charging where it tries to learn your routine and won't fully charge until X amount of time before you supposedly wake up. But they do not have a setting where the phone just stops charging at 80% like Samsung or ay least my Pixel 7a does not.