Rockbox just started out as an itch that needed to be scratched. Namely, the original Archos Jukebox 6000 firmware was utterly awful, so one Bjorn Stenberg took it upon himself to write a much better firmware. Thus, Rockbox was born.
Most of the work was done simply by reverse engineering and reading datasheets wherever possible. A couple of companies, like Austria Microsystems (the manufacturers of the sound chips in the e200 and c200) supplied us with information about those chips, but not every manufacturer has been so friendly.
We did this sort of work entirely on our own, with little input from other companies.