EOS and KDE are basically what SteamOS is running on the back-end. They use Plasma as their DE, and they use Arch as their OS. It's just highly tailored to their hardware since it's all AMD-based and AMD drivers have been open source forever.

There are offshoot projects that build from the SteamOS source and then include Nvidia drivers, but I haven't found one that was as functional as just running Endeavor and customizing it in the way that a Steam Deck would be designed for gaming.