Was curious and checked. It’s immutable and Arch based (similar to SteamOS I suppose):

KDE Linux is an “image-based” OS. This means the entire base OS is provided as one unit, rather than being split up into thousands of packages that are assembled into the final product on your computer.

Sometimes this model is called “immutable” because the base OS cannot be modified in the same way a traditional OS made of packages could. However, this terminology can be misleading; in KDE Linux, only /usr is provided in the form of a read-only image, and everything else on the system is modifiable like on any other Linux-based OS. And /usr is modifiable through the use of systemd extensions.

High-level implementation

systemd-boot is used as the bootloader.

The bootable image is an ISO-compatible .raw disk image built using mkosi. The base system comes from Arch Linux packages, with most KDE content built using kde-builder. Some KDE apps and 3rd-party apps are pre-installed Flatpaks.

/ is a read-write Btrfs volume. /usr is mounted on top of it in the form of an EROFS image/filesystem. Updating the system entails downloading a new one of these and its companion unified kernel image (UKI). 3-4 of these OS images and their UKIs are cached on disk at any given time, providing the ability to roll back in case of problems after updating.

A small helper program updates /etc from the content in /usr/share/factory/etc in the active OS image for anything you haven’t changed yourself. Btrfs snapshots of /etc are generated automatically.

/home is a Btrfs subvolume.

User-facing apps shown in Discover come from Flathub.

No package manager is included, but multiple methods are available for installing software that’s not available as a Flatpak app. This is an evolving topic that’s likely to change over time.

source: https://linux.kde.org/docs/background/