Microsoft developed its own File Manager for Windows 3.0 back in the 90s, and now after two decades, they’ve made it open source. They’ve released the code on GitHub under their official account, and the repository is open to pull requests from all developers.
When Microsoft moved on from command line interface based MS-DOS to Windows 3.0, they needed a GUI based software to manage the file system. For this, they developed a single instance graphical interface File Manager. It was originally a 16-bit program, later rewritten as a 32-bit program for Windows NT.
The code is up on Github repository for anyone fork. You can view the repo here. You can also download the Fiel manager as a binary executable to run on Windows 10.
Learn more about File Manager’s history at Wikipedia.