There is usually a specification for how hardware works. But:

1. The original hardware spec is usually proprietary, and

2. The spec is often what the hardware was supposed to do. But hardware prototype revisions are expensive. So at some point, the company accepts a bunch of hardware bugs, patches around them in software, ships the hardware, and reassigns the teams to a newer product. The hardware documentation won't always be updated.

This is obviously an awful process, but I've seen and heard of versions of it for over 20 years. The underlying factors driving this can be fixed, if you really want to, but it will make your product totally uncompetitive.