You are correct. without datasheets, you have no way to even approach reprogramming a computer system. Those datasheets, schemata, and documentation are some of the closest, most tightly held trade secrets in the tech world.

Increasingly, the world has gotten away from "sell thing to owner to own" to "sell hook for revenue extraction from User". part of which has also increasingly been "be the unofficial or unnoticed arm of power by keeping inconvenient things hard to acquire as determined by the parties in power". Say... industrial equipment without backdoors.

Until I can load my own firmware/microcode, it ain't mine yet. I'd love to have a cobbled together group of heterogenous computing devices just for the novelty.