"Mono is an open source implementation of Microsoft's .NET Framework." [1]
Unity looks like it uses its own fork of Mono, or IL2CPP, which is a fancy thing that converts the intermediate language produced by C# into C++ code, which it compiles. [2][3][4]
[1]: https://www.mono-project.com
[2]: https://docs.unity3d.com/2023.2/Documentation/Manual/overvie...