

It’s using plasma-nano session, which is a minimal Plasma session, and adding a launcher and settings app from what I can see.
You can run it in a regular window if you install the dependencies and use kde-builder to compile it and run. See the Dev docs at https://invent.kde.org/plasma/plasma-bigscreen/
If you do decide to install all the kde-builder stuff, I’d suggest you use a distrobox container to make it easy to remove the many, many packages that it will install in order to set up the build environment
That’s illegal