When Xcode 12 was launched in 2020 it brought with it a new app life cycle for SwiftUI applications. Prior to this, SwiftUI based apps were using AppDelegate.swift and SceneDelegate.swift.

Creating a new app can be done with this code:

import SwiftUI

@main
struct MyNewApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}