id Software is, to no small degree, in the business of making game engines, not just games. Their engines are quite deliberately decoupled from data so they can license them to other companies for their own games.
You'd use a combination of general-purpose tools (Maya, Blender, Photoshop, GIMP), and more specialized tools (like Radiant for map making) to create original media assets.
Custom gameplay is achieved with a combination of a built-in scripting language and modifying a separately-loaded C++ library that encompasses most of what really constitutes the "game" beyond the core graphics engine. The game library appears to be included in the GPL3 release, and was previously available as part of the publicly-available SDK for modders.
You could take this code base and start adding your own custom models, textures, and maps, and have a workable game as-is.
You'd use a combination of general-purpose tools (Maya, Blender, Photoshop, GIMP), and more specialized tools (like Radiant for map making) to create original media assets.
Custom gameplay is achieved with a combination of a built-in scripting language and modifying a separately-loaded C++ library that encompasses most of what really constitutes the "game" beyond the core graphics engine. The game library appears to be included in the GPL3 release, and was previously available as part of the publicly-available SDK for modders.
You could take this code base and start adding your own custom models, textures, and maps, and have a workable game as-is.