Let’s say that I have a stock battery hooked up to the mainboard as well as a USB-C PD battery. The mainboard will drain the USB-C battery first followed by the internal battery. When it’s hooked up to a charger, what happens? Will both the internal and USB-C batteries be recharged?
no, the powerbank probably tells the laptop it’s a Power provider primarily. You would have to alter the USB C PD communication, which is probably hardware and not easily managed by software