Minecraft's obfuscated codebase
Minecraft’s codebase isn’t open source therefore we cannot see how it really works which is fundamental for modding.
The code that runs on our computers is not the source code but is compiled and obfuscated.
In software development, obfuscation is the practice of creating source or machine code that is intentionally difficult for humans or computers to understand
Mod loaders creators had to reverse engineer the compiled and obfuscated code, a very hard and tidious task, to create mappings: a human readble code format that mod creators can use that calls Minecraft native code.
Minecraft Wiki