There's also an optional usb-pd PPS (programmable power supply) where the device can ask for a specific voltage with 20mV granularity & adjust the ask on the fly. This let's devices just pass through power directly to battery while still letting them finely control charge curves. Neat stuff.

But even just adjusting between, I to make up an example, 15V while charging battery, down to 9V after charged, is quite possible. I forget specifics but I almost think I remember there being a minimum amperage draw, so devices canr just ask for 20V then just sip 0.01 amps... They are required to step down to lower voltages if they need so little power. I'm not sure about this though!