And the name of this file manager is Caja. And the desktop is MATE.
This is only one normal GTK-based desktop as for now.
Without bells and whistles, with nice traditional look and feel, and it just works - see
https://static.ubuntu-mate.org/highlights2004.mp4
https://ubuntu-mate.org/blog/ubuntu-mate-focal-fossa-release-notes/
Installable by sudo apt-get install ubuntu-mate-desktop^ .