Some devices provide a hardware level interface for it. It is the case for my ThinkPad T14s and the limiting persists even when changing operating systems.

Some other devices don’t, and the limiting only works while the OS is running. That’s how my previous laptop from ASUS was.

not sure, but it’s probably just an interface to an embedded battery controller and this controller is the one actually determining how/when the battery must charge, so it doesn’t need constant input from the CPU.