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Unity-Technologies/UnityCsReference
Unity C# reference source code.
This is Unity's own dump of the C# side of the Editor and engine — Editor/Mono, the UI Toolkit bindings, build pipeline, asset pipeline, all of it. It's for Unity developers who hit a wall in the docs and want to see what AssetDatabase, BuildProfile, or the animation window actually do under the hood, not for anyone looking to build or fork something.
It's the real internal source, not a decompile or a wrapper — you get to see how bindings.cs files map C# calls down to native, which is the only way to understand a lot of Unity's editor extensibility points. The folder layout mirrors Unity's actual source tree, so once you've oriented yourself once, jumping between related systems (e.g. BuildProfile and BuildProfile/Analytics) is straightforward. It's kept current against the Unity release train (already tracking a 6000.7 beta), so the code matches what you're actually running rather than some frozen historical snapshot.
You can't do anything with it beyond reading — the license explicitly forbids modifying or redistributing the code, and there's no way to compile most of it standalone since it depends on native bindings that aren't included. There's no PR process; if you find a bug you're routed to Unity's separate bug reporter, so this isn't a community-maintained project in any real sense, just a read-only mirror. The tree structure changes between Unity versions with no changelog or migration notes, so a bookmark or deep link you rely on today can silently break on the next release.