Key point
One goal
A demo build is not the final Play Store release. It is a phone-ready APK that a judge, friend, or recruiter can install and see working.
For this track, your proof is simple: the app runs on Android, your main flows work, and your repo shows how you tested it.
APK or app bundle
Flutter docs say app bundles are preferred for Google Play. But APKs are still useful for direct install demos, especially at a hackathon or for a college project review.
So you will build an APK. Later, if you publish to Play Store, you can learn app bundles and signing in detail.