Linux

As others have said, older Steam Machine era SteamOS was Debian-based, Steam Deck era SteamOS is Arch-based.

By default, SteamOS ships with the KDE desktop environment, so if you want a desktop that looks like the one on your Steam Deck, go with a distro that ships KDE.

In terms of terminal commands, most are going to be similar across Linux distros and there will be a non-zero amount of overlap with MacOS. The main differences are going to be in package management, ie installing and removing software. SteamOS has an immutable file system and doesn't let users get to the system package manager (which should be pacman for an Arch-based system) by default; users are expected to install applications via Flatpak.

Flatpak is typically used as an alternate package manager; it's designed to be distro agnostic (doesn't matter if you're on Debian or Arch or Fedora etc.), and really geared toward graphical end-user apps rather than OS components or CLI tools. On a typical Linux desktop, Flatpak would be used alongside the standard package manager. Installing something on your Steam Deck via Flatpak would be done with the same command as on my Linux Mint machine, but I could also use APT to install something from the standard repos. SteamOS prevents users from doing that by default. They chose to do it that way to provide a smart phone-like environment where the underlying OS "just works" and the user just installs games and apps. I keep using the phrase "by default" because you can change that setting and then monkey around with it. Or install a different OS entirely. Paraphrasing GabeN: It's your PC; do whatever you want with it.