A great way to explain this is...actually a newer film: The Running Man. The opening of that film includes a "computer wireframe" of a city. That wasn't produced by a computer at all, but a flat black model with reflective tape on the edges to make it appear as a wireframe when lit in a specific way.
Tron uses a similar trick to create the illusion of circuits of set pieces. Indeed, this same trick is extended to the outfits of the programs themselves.