If you want to get emulation setup on Steam Deck, you need to take a look at EmuDeck which makes installing and configuration nice and easy.
While the Steam Deck from Valve has already proven itself to be a very capable device (just look at our videos!) for modern games, there's a massive amount of classics begging to be played on this handheld. This is where EmuDeck comes in. It's not an emulator itself though, instead it's a setup tool that helps you get other tools and emulators configured properly for the Steam Deck.
So what does it actually do? It will configure RetroArch, provide gamepad configs for Gamecube, Wii, special Steam Input setup for Cemu and Citra and a lot more systems. It will give you a bunch of pre-configured hotkeys, sort out the aspect ratio for certain systems, provide fancy game bezels for retro systems.
Not only that, it also hooks in with an app called Steam Rom Manager to properly sort out all the box art for adding games directly to the Steam Deck Gaming Mode UI, saving you a ton of time messing around with an manual art additions to your Steam Library. It even works directly with EmulationStation to provide everything it needs.
I was going to check it out in a video myself, but I'm not a fan of rehashing what others have done exceptionally well, so here's Retro Game Corps video on it:
It's really amazing to see purpose-built tools and scripts appear for the Steam Deck, which of course will likely work just fine on a Linux desktop too. The Steam Deck continues to be a big boon for Linux gaming as a whole.
Read more and see their guide on the official site.