Does anyone have any experience using AppleUSBUserHCIPort/com.apple.usb.hostcontrollerinterface?

Apparently that seems to be what Virtualization.framework and VirtualHere use under-the-hood to create fake USB devices on macOS.

EDIT: Apparently it's documented API under IOUSBHostControllerInterface.h in IOUSBHost.framework

Turns out it's not actually that hard, I implemented a simple synthetic HID device for macOS:

https://gist.github.com/JJTech0130/fae6b6ee6ae4232172a9188fb199d5d9

Creating fake/virtual USB devices on macOS using IOUSBHostControllerInterface

Creating fake/virtual USB devices on macOS using IOUSBHostControllerInterface - ConsumerKeys.swift

Gist