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.

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

You’ll need to generate an App Store Connect API Key
with App Manager access to automate operations with App Store Connect. To make
subsequent commands more concise, set the following environment variables from
the new key: issuer id, key id, and API key file.