7.5W vs 15W chargers vs USB port volts x amps

Okay, so a timely article dated 1/09/2020 on wireless charging (https://support.apple.com/en-us/HT208078) refers to charging iPhones at 7.5W but still leaves me confused after studying up on how watts = amps x volts.  

I just bought a Letscom brand 15W wireless charger that is connected to a “1 amp USB self-adjusting” port at the back of a new LaCross Projection Weather Alarm Clock.  I assume “self-adjusting” means the voltage is adjusted, but I don’t know what range of voltage is available from that port.

Now, if I’m wrong and the voltage is actually fixed at the standard 5V for USB, then at a max of 1 amp the charger would only receive 5 watts, and my iPhone X is probably charging slower than it could.  But if the port can output up to 7.5V then at 1 amp my new charger is possibly receiving only 7.5W from the port effectively performing the same as an Apple brand 7.5W wireless charger.

But what happens if the 1 amp clock port can provide 15 volts, allowing the charger to perform at its full 15W rating?

Bottom line, with the QI wireless charger standard, is the iPhone X telling the charger to draw only 7.5W, regardless what is available from the USB port, or will it accept up to the full charger rating of 15W as long as the charger can draw that amount of power from the usb port?