Is there an existent method, a driver, or some software that facilitates direct USB-to-USB networking in macOS similar to FireWire-to-Firewire ethernet or the new Thunderbolt-to-Thunderbolt ethernet networking?
There's a number of USB to USB direct connection cables out there. Here's one example from StarTech: https://www.startech.com/Server-Management/KVM-Switches/dual-system-swap-cable-kvm-switch~SVUSB3DSA
Another example from LapLink (look at the USB 3.0 cable): https://web.laplink.com/cables/
My guess these cables and many more like them are nearly identical, all using the same PL27A1 IC from Prolific Technology: http://www.prolific.com.tw/us/ShowProduct.aspx?pcid=43 The chip emulates a serial or Ethernet connection but at 5 Gbps. It's exact behavior depends on how the chip is configured and what software/drivers exist on each computer. My guess is, given what I've seen from reviews, that most operating systems include support for this chip in providing an Ethernet-like network.
There are cheaper passive USB-A to USB-A crossover cables that meet the USB 3.x spec for data transfer between USB hosts, like this one: https://www.datapro.net/products/usb-3-0-super-speed-a-a-debugging-cable.html In the description they call it a "debugging cable" since as of today (to my knowledge anyway) that's all any operating system allows it to do. There's no reason someone can't write drivers for this cable to create a network between two computers. That is except the large number of similar cables that violate the spec and could open up the authors of such software to all kinds of criticism because people bought the wrong kind of cable and broke their computers.
USB-C to USB-C, and USB-A to USB-C, cables are generally safe to connect two computers since there's been a crackdown, by some big players in the industry, on those selling noncompliant cables. The only thing I can see stopping these cables from making a network connection is an apparent lack of need for it, at least from the view of the people that make operating systems for desktop computers. I'm seeing questions over USB to USB connections often enough that I expect this will likely change soon.
In reading things on the new USB4 spec a host to host connection will be supported. Well, it's technically already supported since it's been in the USB 3.0 spec for some time now. It's only that this time the feature will be better defined and/or support is mandated. I have to wonder if this will trickle down to existing USB 3.x devices. There's not a lot of difference between the old USB 3.x and new USB4. Like all USB versions before it USB4 requires backward compatibility.
As far as I know every Apple Mac computer with a USB-C port will support "Target Disk Mode" on those ports. This allows a Mac to boot into a mode that makes the computer a very expensive USB drive. This is helpful in a number of situations. This proves that the hardware is there for a Mac to act as a USB device to another computer. It's just not a feature supported/allowed by the operating system.