What is the difference between the App, Window, and Page objects?

App (Application) is the singleton root of the app and creates the initial Window. A Window represents an OS window/screen host and owns a navigation root (its Page). A Page is a single screen of content. Desktop platforms can open multiple Windows; mobile typically uses one. Application.Windows exposes all open windows.

public partial class App : Application
{
    public App() => InitializeComponent();

    protected override Window CreateWindow(IActivationState? state)
        => new Window(new AppShell()) { Title = "W3Teacher" };
}