Technical perspective first…

This is Unreal Engine 1, which used UnrealScript programming language. It was extremely flexible, and you can extract the original UnrealScript code (including comments) from the game. This means it’s nearly an open source game, except for the native code. But pretty much everything is controlled by the UnrealScript anyways. Including the GUIs, HUDs, conversations, most of the AI stuff, damage calculations, keyboard key bindings, etc.

On top of this, Deus Ex released their SDK tools (I think in 2001, around the time of the multiplayer patch). Which is their version of the UnrealEd map editor, conversation file editor, and UnrealScript compiler/extractor.