The IBM PC spec was included with the IBM PC (or sold for $100 or so). Schematics, BIOS source code, everything. IBM was meticulously documenting everything, and that was actually common and expected at the time (e.g. I had the same info about my ZX Spectrum; I remember the same info was available for the Apple][ and C-64 at the time).

A defining moment was when the "clean room" approach to implementing a BIOS was certified non infringing by courts. Clones were available (as in, 5 years or so) before that moment, but the BIOSes had a cloud of copyright uncertainty.