A small correction on USB PD… It’s not just USB PD that supports power delivery: Standard USB from way back in 1.0 also supports power delivery to devices as standard, but it’s only up to 100mA in USB 1.0, 500mA in USB 2.0 and 900mA in USB 3.0, all at 5V.

USB PD is a dedicated power delivery USB protocol that supports much higher currents (up to 5A) as well as dynamically configured voltages (so, not fixed as 5V anymore) though it’s all negotiated so your 5V-only phones isn’t going to just get burned with 20V from a USB PD charger.

Since Power = Current * Voltage USB PD can put out quite a lot of power for supporting devices (the maximum depending on what both sides support), which means much faster transmission of power via USB which for example means faster charging of devices via USB with USB PD.

Anyways, the point being that even really old USB 1.0 can charge your device (just really really slow), and that standard charging speed dies go up with each new Standard USB generation (so a standard USB 3.1 charger without USB PD support can still push a nice amount of power down the line to charge devices), but with USB PD things really take off though only up to a shared maximum that both sides support.