Open the Main.storyboard file by clicking on it. This file contains the UI elements. At the moment, it only contains an ARSCNView, which occupies the whole screen.
If you right-click on the scene view, you will see the Referencing Outlets that are linked to the View Controller with the sceneView variable, as shown in the following screenshot:
The UI element ScenView and its Referencing Outlets
Let's take a closer look at the view controller element.