Do they just have a very abstract and capable API? some mods for some games can be pretty complex.
In: Technology
Most games do not store their game data the same way they store their code. So it is possible to reveal the format of the game data and not the code. This makes it possible to change or add textures, 3D models or even complete levels. Some games might even have systems for the NPCs and scripted events in an easy format that can be changed. This may have started off as a way for the level designers to work independently from the coders but have the effect that third parties might also change the game content.
A lot of games is also written in a way that makes it relatively easy to reverse engineer the source code. Languages such as Java and C# have the disadvantage that the resulting compiled code is relatively easy to decompile. You may not have the variable names and the exact structure which makes it a bit hard to work with but it will allow you to make changes and test them out.
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