How do usb-c connectors carry 100W at 20V without melting?
If you look at the American Wire gauge reference you can see that a 24 AWG of 1.5 mm diameter can withstand 5 Amps. However, becauce of RI^2 loss they will likely use a bigger wire,  Based on the teardown picture below, I'd say they probable use 20 AWG. 

You only need 2 of these wires, one for Vcc and the other for ground. They can easily fit in a USB-C cable.

https://www.multicable.com/resources/reference-data/current-carrying-capacity-of-copper-conductors/

https://www.mvdesignlabs.com/tear-down-tuesday/teardowntuesday-of-a-usb-c-cable/

the 4 VCC pin are connected to the red wire which seems good enough to withstand 5 A (probably more). The 4 GND pins are connected the black wire.