Until now, I had been under the impression that KDE was just arch Linux itself.

Like others have already noted, KDE Plasma^[1]^ is widely available and thus not only limited to Arch Linux. Heck, the same applies to 99% of the available software on Linux; universal package managers^[2]^ have been vital to this.

Would you happen to know a good way for me to learn more about Linux, and how to put it to good use from a beginner’s perspective?

As you already own a Steam Deck, I assume you want to look into how you may improve your mileage out of it. Others have already noted how you may do so for more traditional systems. But the way Linux is utilized on the Steam Deck is rather unique. It utilizes immutability^[3]^ (i.e. the inability to make certain (permanent) changes) which makes it rather harsh to change certain parts of the system; SteamOS' implementation might even require you to redo some of these changes every so often... which is probably not what you were expecting. To circumvent this, perhaps it's worth exploring other SteamOS-like distributions that are more friendly towards tinkerers. There are many to choose from; perhaps this breakdown may help you with making an informed decision (even if it's found on a page dedicated to the Legion Go).

1. That is, the desktop environment (i.e. the piece of software responsible for how you visually interact with your system) that team KDE works on. They're also responsible for many other projects; like Kate, Kdenlive and Krita etc (these are often easily recognized by their names that start with a "K").
2. We may refer to package managers as the original App/Play Stores; a piece of software used to find, install and upgrade software. For a long time, every major distribution (like Arch, Debian and Fedora) had its own repository (i.e. set of installable software through the package manager). This meant that, it was very conceivable that software may be packaged (i.e. distributed and maintained through the repository) on some distros (abbreviation for distributions) but not on others. In the last couple of years, so-called universal package managers (like AppImage, Distrobox (technically this doesn't belong here, but it does allow access to packages found on (other) distros), Flatpak, Guix, Nix and Snap) have become alternative package managers that are distro-agnostic. And have slowly, but surely, ridden Linux distros from concerns related to package availability.
3. There's a lot to say about immutability. But for now, it's most important to note that not all systems that are (sometimes falsely) referred to as immutable are created equally. For example, the respective implementations for Bazzite, Jovian NixOS and SteamOS differ immensely from one another. Arguably, referring to Bazzite and Jovian NixOS as immutable with 'unchanging' being what's implied, would be a major disservice to both projects.