In the game handlers hub you will be able to explore all the available Nucleus Co-op handlers very easily and download the handlers directly without using the app, you will also be able to see the handler code, its version history, extra information related to the handler and game and give each handler points (hotness).
Remember to only download handlers that have been verified by the SplitScreen.Me team as fully safe (you will see the verified green icon).
Most handlers are made and tested using the latest legit Steam version of games, so you need the Steam version unless the game handler notes or description mention it works with other versions too (GoG, EGS). Nucleus Co-op also supports some fan games and open source games. Xbox Game Pass games are currently not supported because of their DRM.
Most game handlers work fully offline as the game instances connect via real or emulated LAN, only few handlers require internet connection to connect the instances either because they connect to a game own servers or because they require multiple accounts.
Do you not see the game you want to play in the list of supported games? Request it or check out this guide on how to create a handler for that game yourself. For more info on handlers, see what are handlers?.