Discover
App Store(Flatpak Store)
All of the apps that are on this page can be found in the "Discover" app that comes with the Steam Deck. It can be found on the Desktop Mode of the Steam Deck.
Chiaki
Playstation Remote Play
If you have a PS5 and you want the ability to play your PS5 games on your Steam Deck, Chiaki is what you are looking for! It is an app that configures PS Remote Play for you to use on your Steam Deck.
Lutris
Install Windows Games/Launchers
Lutris is a free and open source game manager. For Steam Deck purposes, it will be so that you will be able to run "Windows Only" games.