Structures with special mob spawns (1.12)

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Hi, I want to make a mod in 1.12 where you can create a structure in game (like creating a village) that spawns specific mobs within the structure (like a witch hut). I cannot however, find the code that governs these processes, can anyone point me in the right direction to look? For example a class name that contains the relevant code, a mod previously created that does something similar, or really any advice would be greatly appreciated. Thanks in advance!

[Lumby]

My goal is to basically be able to place down a special modded block and create an area that only spawns my custom mob