It's not just the tensions. The USB standard provides for a constant 5V and this voltage is generally present at each connector. Only after connecting the supporting device of Quick Chrge or something similar does the phone communicate with the charger which increases the voltage. More specifically, the phone controls the charger. I used to look for how it is done with Qualcomm. Here is the charger diagram for Quick Charge 2.0:
The layout on the left is almost a textbook example of how converters work in low-power chargers. On the right, there is an integrated circuit that communicates with a phone connected to USB. This ok determines when the inverter on the left will turn off, so it determines the output voltage. And here the chip from one manufacturer may not get along with the chip from another phone manufacturer. As far as I know, the charge regulator is responsible for it.
