Release build can be made using flutter build <options> apk command.
Before that create a key.properties file in the android folder. Add folloiwng lines to that file:
keyAlias=<your keystore alias>
password=<your keystore password>
storeFile=<path to the keystore file>
Then run flutter build appbundle -t lib/main_prod.dart --flavor prod to generate app bundle (.aab) file.
To generate .apk file, run flutter build apk -t lib/main_prod.dart --flavor prod.