USB-C to 15V conversion
You should check the USB-C standard.

USB-C by default provides 5V, you need to communicate with the supply source through the CC line to get higher voltage.

I think it would be quite complex to do it and the best way is probably to use a chip that does it for you. Unfortunately, since USBC is relatively new, it might be difficult to find.

Given that, also it wouldn't work with all usb-c supply, you would need a supply that is able to supply 15V, which is probably not the case for all.

All that said, if you want 15V off a USB supply, as long as the power is enough, it would be easier to use 5V with a boost converter to reach 15V.