Set up code signing.
    
      
 Follow the Android app signing steps.
      
 On iOS, create and sign using a
distribution certificate instead of a development certificate when you’re
ready to test and deploy using TestFlight or App Store.
        
          Create and download a distribution certificate in your
Apple Developer Account console.
          
open [project]/ios/Runner.xcworkspace/ and select the distribution
certificate in your target’s settings pane.

Create a Fastfile script for each platform.
    
      
 On Android, follow the
fastlane Android beta deployment guide.
Your edit could be as simple as adding a lane that calls
upload_to_play_store.
Set the aab argument to ../build/app/outputs/bundle/release/app-release.aab
to use the app bundle flutter build already built.
      
         On iOS, follow the
fastlane iOS beta deployment guide.
You can specify the archive path to avoid rebuilding the project. For example: