Application Scene Delegates support in RN >= 0.74

Description

When we added CarPlay support to our React Native app we needed to switch from App Delegate to Application Scene Delegates.

Independently of whether the app was started on the phone (PhoneScene) or on the CarPlay-client (CarScene), the first code to run natively will always be the AppDelegates application:didFinishLaunchingWithOptions: method.
A React Native app usually calls the super-method in its AppDelegate, which is implemented in React Native's own RCTAppDelegate. The problem with this is that RCTAppDelegate assumes a phone usage and creates a rootViewController along with a window for the app to be displayed in. This leads to problems when launching the app on the CarPlay-client first, since CarPlay does not require a rootViewController or a window to display its views.

The key to solving this problem is to split the app initialization logic into PhoneScene and CarScene (which are both subclasses of UIResponder) and only run the code required to set up the React Native bridge in the AppDelegate. We can achieve this by not calling the super-method in application:didFinishLaunchingWithOptions: but instead create and call a
custom init method.

Prior to React Native 0.74 this wasn't a problem, since all methods needed for setup were publicly exposed.
Starting with React Native 0.74, the root view is created via RCTRootViewFactory with no way of instantiating one from the custom initialization routine in App Delegate.

How do you plan to support Application Scene Delegates in the future?
Are there any options to create a RCTRootViewFactory without patching the header file as described here?
Would it be problematic to expose createRCTRootViewFactory in the header, making it accessible from the App Delegate?

Steps to reproduce

Try setting up a RN 0.74 or 0.75 app via application scene delegates

React Native Version

0.74

Affected Platforms

Runtime - iOS

Output of npx react-native info

irrelevant

Stacktrace or Logs

none

Reproducer

none

Screenshots and Videos

No response

Start with RCTAppDelegate.mm, especially createRCTRootViewFactory, and the AppDelegate application:didFinishLaunchingWithOptions: entry point. Review the PhoneScene and CarScene initialization described in the issue, then clarify the supported public API and acceptance criteria with maintainers. Done should provide a supported React Native initialization path for Application Scene Delegates without requiring an inappropriate phone root view or window.