Many languages have their compiler written in themselves. The first compiler, the bootstrap compiler, might be written in a different language. Once the bootstrap compiler is complete, however, the language compiler can be compiled by itself, and from then on, each new revision of the compiler can be compiled from the previous version. This is an example of Self-Hosting, which may refer to any software which is used in its own development.
Gits history is tracked in git, windows development is done on windows, and the open-source, C# compiler—Roslyn—is written in C#.