There’s a host app that runs on the host machine alongside Sunshine that reads your Steam library and the Deck plugin adds an icon on each game’s banner on your Deck. When you click the icon, the plugin communicates with the host app and then automatically starts a Sunshine session that then starts up the game you were on. You only have to add one “app” to Sunshine and set up the MoonDeckBuddy app on the host, and then you have streaming for your entire library available.