I've played more PC games now than I have in the last 10 years.
Valve launched a Linux-based gaming platform called Steam OS in the early teens. Other companies made the hardware, and games had to be compiled for Linux - this was before Wine/Proton was good enough for gaming. It was open sourced, but stopped getting updated around 2018. SteamOS 1 and SteamOS 2 have public repos.
The Steam Deck launched with SteamOS 3. Many parts of it are open sourced (including gamescope-session, which lets you skip the window manager and go straight to the Steam UI), but there's no official "SteamOS 3" repo. There's a repo of unknown propriety called "evlaV" that claims to be a mirror of the official SteamOS 3 one. It's unclear where it comes from or how true/complete it is. https://gitlab.com/evlaV
SteamOS is an "immutable" Linux, which is to say that the system files are read-only. This allows ChromeOS-style updates without worrying about how someone has customized their system. It uses an application format called "Flatpaks" which skips the whole deb/rpm packaging debacle by bundling all the dependencies in a distro-agnostic container. It's a bit like how a MacOS .app folder includes everything the app needs to run.
There are also lookalikes that implement the Steam Deck UI with open source projects, like mimicking the SD's PIN unlock.
-----
Valve's main competitors in the portable PC space are Lenovo, Asus, and a bunch of Chinesium also-rans. They all ship with Windows.
There's also a vibrant community of people who buy them for the hardware, and try to replace them with as-close-as-we-can-get-without-official-repos SteamOS 3. They include:
- Bazzite: an immutable version of Fedora that uses GitHub Actions to "layer" packages on top. Basically, you write some markup that says what packages you want, GitHub mints an image, and that gets installed on your handheld. When a new version of Fedora/Bazzite comes out, your layers get rebased on top of it. (The whole immutable Linux thing is heavily influenced by Git.). Fedora is used for compatibility, but the gaming pieces (copied over from evlaV) run in an Arch container by way of Distrobox.
- Nobara: Fedora, as personally customized by a Red Hat employee to be better for gaming. His handle is GloriousEggroll, and he's also a prolific contributor to the Proton API compat layer that allows DirectX games to run on Linux.
- Jovian-Nix: NixOS, configured to run the SteamOS/evlaV packages on top
- Holo: The SteamOS equivalent of a Hackintosh: some guy in Russia modifying SteamOS to get it to work on a non-Steam-Deck.
- ChimeraOS: a distro designed for set top gaming and handhelds, derived from Arch.
But the UI; how I actually interact with the system? I care about that.