It's not just a portable PC. It's a portable retro console wonderland.
Holding down the power button on the Steam Deck brings up a small menu that includes a "Switch to desktop" option. This leaves behind the SteamOS interface and plops you down on a classic desktop running on top of Arch Linux. I started looking up terminal commands to install programs in Arch before finding out I was completely wasting my time. This desktop comes with a pre-installed app store just like macOS and Windows (except everything here is free). It's called Discover, and it's conveniently pinned to the taskbar.
And all the emulators I wanted are in there, installable with a click:
- Dolphin, for GameCube/Wii
- DuckStation, for PlayStation 1
- PCSX2, for PlayStation 2
- PPSSPP, for PlayStation Portable
- Yuzu, for Nintendo Switch
- bsnes, for the Super Nintendo
There are some other emulators available in Discover, too, including mGBA for the Game Boy Advance and Citra for the 3DS. The best bit is that these aren't just one-off executables you're installing—when new builds of the emulators are released, you can update with a single click, too. It's honestly a godsend. Because of the way Valve set up the partitions and file permissions on the Deck, installing emulators or any other software via the terminal is pretty locked down unless you have some serious Linux skills.
Once I had my chosen emulators installed, I launched the desktop version of Steam and used the "Games > Add a non-Steam game to my library" menu to add each emulator to Steam so that I could access it from the SteamOS interface. (One note here if you're also installing Duckstation: it has two interfaces, and "DuckStationNoGUI" is the one you want to add to Steam. It's built to play nice with controllers.)
Read about the full installation process here