I've been a developer for over a decade, and reading code like this still leaves me in awe...
In the beginning it wasn't (usually) one person but an entire team studying every component of the console, dumping the BIOS and then coding something that (at first) could at least boot the game. After that it was mostly about working out the details like graphic glitches, slow framerate, certain graphical issues, etc...
But once the "core" was made other teams or individuals could use it to make their own versions, or port it to other OSes/architectures.
The NES is together with the Sega Genesis one of the most emulated consoles out there: you'll find emulators of these two for every computer, PDA, PMP, phone or tablet.
