For microcontrollers, tinyusb seems to be the place to be. It supports a lot of architectures. The Pico RP2040 has tinyusb as part of its SDK. I've actually got something working, and I was toying with the idea of making the Pico controllable over USB. Once you can peek and poke memory addresses on the Pico, the world's your oyster.

Tinyusb seems quite flexible. You can write anything. It seems to have quite a bit of "canned" functionality, too, so if you wanted to write a keyboard host, for example, it's a case of trying to work through one of the demos and adapting it to your needs.

The problem with USB is that there is a lot of assumed knowledge as to how it all works. I only started playing with it recently, and it feels like being parachuted into enemy territory without a map. I wrote some of the basic here: https://medium.com/@oblate_24476/introduction-to-usb-from-a-...