DOOM draws very little. An entire level is 3000-5000 polygons and on the screen, it rarely draws more than 1500 (polygon based ports)

Whatever it's architecture is, porting in is trivial because it's not a hard problem.

It's a great game and its geneious is undeniable. But porting a 1993 game is not really going to tell you anything interesting about "How code structure survives across 44 ports".