I thought for a moment Microsoft had uploaded some new Windows Explorer source code and got excited... I guess that tells you how different an era we live in. It wouldn’t be a huge leap.
The screenshots look nice, though. I can’t say I am really a huge fan of UWP UIs on desktop... but this looks alright.
I thought so too, given that MS has already released the source of the original File Manager:
https://github.com/microsoft/winfile
...and it compiles to a nice small and fast ~300KB executable, in contrast to their open-sourced and significantly less efficient UWP calculator:
https://github.com/microsoft/calculator
I believe the old leaked Win2k source has much of the explorer.exe source, if you're really curious.