For apk (Android) you need to run the command :
flutter build apk --release
If you want to split the apks per abi (Split Apk) then run
flutter build apk --target-platform android-arm,android-arm64,android-x64 --split-per-abi
For ipa (iOS) you need to run the command :
flutter build ios --release
From the console
P.S. --release is optional as it is by default
if you need debug build, just replace --release with --debug
you can find the released APK or IPA files form
build/app/outputs/flutter-apk/app-release.apk