With Thunderbolt and operating systems that support Ethernet over Thunderbolt, a virtual network adapter is automatically configured for any Thunderbolt connector, so connecting a USB C cable between 2 such computers should just work, as if they had Ethernet 10 Gb/s connectors.
With USB 3 USB C connectors, you must use USB network adapters (up to 2.5 Gb/s Ethernet).
It merely requires one side to be capable of behaving as a device, with the other side behaving as a host.
I.e., unlike PCIe hubs, you won't get P2P bandwidth savings on a USB hub.
It just so happens that most desktop xhci controllers don't support talking "device".
But where you can, you can set up a dumb bidirectional stream fairly easily, over which you can run SLIP or PPP. It's essentially just a COM port/nullmodem cable. Just as a USB endpoint instead of as a dedicated hardware wire.
It's very useful for sending large data using minimal equipment. No need for two cat6 cables and a router for example.
Secondly, I'm not sure if a crossover cable setup will autoconfigure the network, as the poster above says, it has been since the 90s when I bothered trying something like that!
You don't need crossover cables anymore. You can just connect a regular patch cable directly between 2 devices. Modern devices can swap RX/TX as needed.
As for auto-configuration, that's up to the OS, but yeah you probably have to set up static IPs.
I was under the impression you have to set up the devices as each other's default gateway, but maybe I'm the one not up to modern standards this time.
Therefore they can talk directly, without using a gateway.
Their corresponding Ethernet MAC addresses will be resolved by ARP.
The problem is that in many cases you would have to look at the autoconfigured addresses and introduce manually the peer address in each computer.
For file sharing, either in Windows or using Samba on Linux, you could autodiscover the other computer and just use the name of the shared resource.