Intro to USB drivers for Plan9 and 9Front
invidious.tiekoetter.com
Part 1 on writing drivers for USB devices. I'm using 9Front, but some of this will apply to legacy Plan9 too. Link to the OpanBSD code I was looking at; https://github.com/openbsd/src/blob/master/sys/dev/usb/if_urtwn.c

As someone who has encountered unsupported peripherals, this quick look into how drivers work was fascinating to see.