The 'code' probably is just assembler instructions. You can't create a game on a limited system like the NES in C#. It has only 2k of memory, so you have to do everything yourself.

There is no way to port such a game with it feeling authentic, other than the emulator.

This isn't exactly true. Games from as early as the Amiga era were known to have been ported by interpreting the gameplay code from the lead release platform(typically, an 8-bit computer or the Atari ST which in its early years put up good competition for the computer gaming crown) and rewriting the I/O-sensitive stuff. These were not considered great conversions at the time since the result was often sluggish and couldn't make proper use of the available hardware, often just looking like the original but with a different palette and worse sound, but they got the job done very quickly and accurately.

that's true, code is just what's in the programmer's head and the fingers are the compiler for that kind of machine.