USB PD Explained

USB Power Delivery is a negotiation between charger and device so the phone takes only the power it asks for.

USB Power Delivery (USB PD) is a standard over USB-C. The charger advertises profiles. The phone or laptop requests a voltage and current. Charging starts only after that handshake.

That is why a 100W charger does not force 100W into a phone. A 20W iPhone still charges at iPhone rates when the cable and charger both support PD.

USB-A ports do not carry USB PD. A USB-A to USB-C cable from a USB-A wall port cannot do PD, even if the phone is USB-C.

USB-C charger + USB-C cable = PD is possible.
USB-A charger + USB-A cable = older charging, not PD.
The limiting piece is always the weakest of charger, cable, and phone.