when do you start using the swiftui interface with the uikit life cycle?

With SwiftUI you should in most cases use SwiftUI lifecycle.

If you need some AppDelegate methods you can use @UIApplicationDelegateAdaptor:

• SwiftUI app life cycle iOS14 where to put AppDelegate code?

There are some cases, however, when you need UIKit lifecycle:

• access to SceneDelegate (see Is there any way to call SceneDelegate methods in iOS 14 app life cycle?)
• deployment target iOS 13 (SwiftUI lifecycle is available on iOS 14+).