How can I download my app package directly from app store connect?
The title and body ask two different things, ‘app package’ and ‘original files’.

The ‘original files’, source code, is never uploaded to Apple. You will need to get these from the original developer.

The compiled ‘app package’ in an .ipa bundle for execution on an iOS device (not Simulator) cannot be downloaded from App Store Connect.

Apple does not provide any way of retrieving an upload app/IPA. It's neither possible through App Store Connect nor the App Store Connect API.

https://stackoverflow.com/questions/60947606/download-ipa-build-via-app-store-connect-api

If you want to run the app on a physical device without releasing to the App Store, add the app to TestFlight and enable internal testing.