To add a game, ensure the Backbone app is active before returning to the home screen to launch the relevant game, once the game is launched then simply press the Backbone button and this will add the game to your Backbone launcher. This can also be done for certain services, such as Xbox and PlayStation Remote play.
To remove a game, the game must still be installed prior to removing from Backbone app. Ensure the Backbone app and the game you want to remove are launched, then go to iOS Settings - iPhone Storage - Select relevant game - Select Offload App. Then return to Backbone app and select A with the relevant game highlighted, this will then remove that selection. You can view a video outlining this process here.