He disassembles the .exe and spends hours upon hours digging through assembler language, reverse engineering 'chunks' of it at a time, handwriting that into human readable code. It certainly isn't an automated process, and it definitely can't be a fast/enjoyable process.
A heavily tooled engine with a closely tie-ed in scripting system and model/asset manager would do you much better in the long run. Best case scenario: you have approximated very old and provably buggy code.
Using modern formats means easy usage in modern day software.
Just my opinion though. If you are hell bent on manually parsing assembler language, you might want to get a PhD in asm first
Edit: Unity offers a free version of their stuff for personal use -->
https://store.unity.com/products/unity-personal