I wonder if I could migrate data between two Windows computers with a USB-C cable (but not one of those unusual cables with a knot in the middle, just a normal USB-C cable) and with Windows default software (not any third party software).
Would that be faster than using a crossover Ethernet cable?
Cut and Paste from Superuser that answers that question.
To connect one Type-C device (one PC) with another Type-C USB device (or another PC) and expect some connectivity, at least one of the "Type-C link partners" must support so-called DRD - Dual Role Device. The DRD port advertises its dual role by continuously switching its CC (communication channel) pins from 5.1k pull-down (signifying a USB device) to 56k-22k-10k pull-up (signifying USB host with different VBUS supply capability). It does this flip-flop several cycles per second.
However, to be a DRD Type-C device, it must have TWO USB controllers inside, one of xHCI (host controller interface), and another of "DCI" type - device controller interface. The IO of these two controllers must be multiplexed at the USB port pins. Currently only a few products (notably the Intel SoC aka "atom cheery trail" family, and other mobile-oriented chips found in mobile phones) have this capability. If a PC is made of desktop line of processors, no DRD is available yet.
If both PC are of the same kind, no connection (and no harm) will happen.
If one Type-C PC has DRD functionality, it will pick the phase of its "flip-flop advertising" with the role that is opposite to the connected single-role device. If the connecting device is host, the DRD device will lock as device, and vice-versa. If both devices are DRD, the roles will be selected at random, and later should be switchable in software.
Is it faster than Ethernet? Both have the same possible max transfer speeds of 10G per second but chipsets, driver support and any complications from windows itself may dictate the actual speed