You can follow the status of your build in the
Activities tab of your app’s details page on
App Store Connect.
You should receive an email within 30 minutes notifying you that
your build has been validated and is available to release to testers
on TestFlight. At this point you can choose whether to release
on TestFlight, or go ahead and release your app to the App Store.

For more details, see
Upload an app to App Store Connect.

Create a build archive with Codemagic CLI tools

This step covers creating a build archive and uploading
your build to App Store Connect using Flutter build commands
and Codemagic CLI Tools executed in a terminal
in the Flutter project directory. This allows you to create a build archive
with full control of distribution certificates in a temporary keychain 
isolated from your login keychain.

Install the Codemagic CLI tools:
    
pip3 install codemagic-cli-tools