18.5 Summary
SwiftUI applications are constructed hierarchically. At the top of the hierarchy is the App instance which is responsible for the launching and lifecycle of the application. One or more child Scene instances contain hierarchies of the View instances that make up the user interface of the application. These scenes can either be derived from one of the SwiftUI primitive Scene types such as WindowGroup, or custom built.
On iOS or watchOS, an application will typically contain a single scene which takes the form of a window occupying the entire display. On a macOS or iPadOS system, however, an application may comprise multiple scene instances, often represented by separate windows which can be displayed simultaneously or grouped together in a tabbed interface.
