RetroNX is a version of RetroArch ported to the Nintendo Switch by m4xw and the RetroNX Team. It is a homebrew app used to run emulators and retro games while using custom firmware. See below for links to navigate the wiki.
Discord Server: Link
RetroNX splash screen and menu. Credit: ProtoPropski
Here is how to setup RetroNX on your Nintendo Switch.
Here is how to change the look of RetroNX to your liking.
Here is how to create playlists using Windows.
Here is a list of supported cores (emulators).
Here is the RetroNX FAQ.
Here are the official libretro docs (wiki), where you will find more info on RetroArch and it's specific cores.