What is the best resource to learn about USB protocols?

"USB in a Nutshell" [1] was really useful to me. As far as tooling goes I'd take a look at the "usb-tools" project [2] on Github. The GreatFET One hardware [3] is good and works with the Facedancer software hosted by the usb-tools project.

I enjoyed working on USBQ and learned a lot by doing it but the kernel loadable module it depends on wasn't written by me and probably needs some love. The usb-tools project and the related hardware has advanced quickly. Were I to do the same project over again (hacking Zwift for fun) I'd probably start there.

There is so much to USB... even some surprising things! [4]

[1]: https://www.beyondlogic.org/usbnutshell/usb1.shtml

[2]: https://github.com/usb-tools

[3]: https://www.greatscottgadgets.com/greatfet/one/

[4]: https://link.springer.com/chapter/10.1007%2F978-3-030-28752-...

When I looked for resources about USB, this website is recommended frequently: https://www.beyondlogic.org/usbnutshell/usb1.shtml

I see references to a hardware board but nothing detailing the specifications or where to buy one. Would any computer work as the MITM device, perhaps a Raspberry Pi?

I used the BeagleBone Black. There is a loadable kernel module which, lucky for me, just worked. The kernel module is from the USBiquitous project [1].

See my other comment about recommending starting with usb-tools / Facedancer.

[1]: https://www.sstic.org/media/SSTIC2016/SSTIC-actes/usb_toolki...