csrss.exe

Windows executable Reference

An .exe file is an executable program on Windows. The file has the Portable Executable format and contains machine code, resources, and metadata. The Windows loader reads the file and starts the program. A user or a different process can start it directly. Applications, tools, and installers ship in this format.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.