There's a lot of information online, of course those are not the only places to get it.
Most of the times, to create an emulator people search for documentation of the CPU or any other piece (if you open a NES console you can see the processor model and you can search for the instruction set). If there's no documentation for a certain piece, they reverse engineer the hardware. I'm not sure if it's legal or not but I guess it's legal, as long as you don't create the same thing with that knowledge. Another source of information could be the emulator's code itself, if it's accurate you could get some details there.
The thing is that reverse ingineering a piece of hardware is HARD, even understanding the documentation could be a hard task. That's why emulators take years to be developed, and even the most advanced ones have issues with some games.
Anyway, why would you create a console with outdated hardware? You can create a console with current hardware and it will probably be a lot easier. Understading an old hardware could be fun, and could be usefull to write an emulator or a compatible console, but I don't see the advantages of using that knowledge to create something new.