https://arstechnica.com/gadgets/2018/04/relive-the-1990s-on-windows-10-with-the-newly-open-sourced-file-manager/

Back in the early prehistory of desktop computing, we didn't have Explorer for managing our files in Windows. We had File Manager. Explorer scatters windows liberally across our desktops—we're still waiting for the tabs to organize the clutter—but File Manager never had that problem. File Manager came from an older time, the era of the Multiple Document Interface (MDI) wherein one File Manager window contained multiple separate panes for actually browsing your files.

File Manager continued to ship with Windows long after it was replaced by Explorer just in case somebody needed it, but for the last decade or more, Windows hasn't included it. To remedy this, Microsoft has released the source code of its venerable file management application under the MIT open source license.