Basically everything is known about the Amiga - people have reimplemented compatible hardware on an FPGA (https://en.wikipedia.org/wiki/Minimig). Every piece of code that runs from power on has been examined in detail (there's even a free software from-scratch version of the boot firmware). The Pi still requires closed boot code that runs on the GPU in order to bring up the application processor, and that still isn't well understood.
We know how the platforms work in general, but 386-era is when SMM appears for the first time and so it's unlikely that anybody has full knowledge of the entirety of a specific system.
