Create usb drives you can boot multiple operating systems off of
that’s super easy to do with a million and one tutorials but sure go for it.
now… how does one make a usb drive or have the firmware?
“now … how does one make a usb drive … ?”
One possible starting point: http://www.cypress.com/file/138911/download
Looks like it handles all of the physical layer and most of the link layer.
Just add your favorite memory and write your own application firmware.
Probably a whole lot easier and cheaper just to take an RPi Zero and make it behave like a USB memory stick.
something like https://null-byte.wonderhowto.com/how-to/make-your-own-bad-usb-0165419/ comes to mind