APK

An APK is the installable unit of Android: a ZIP archive containing compiled Kotlin and Java code (classes.dex), resources, the binary AndroidManifest.xml, native libraries under lib/<abi>/, and in a React Native release build the JavaScript bundle under assets/. Every Android app on a device was installed as one or more APKs.

For React Native developers APKs are the everyday testing format, because they install directly:

cd android && ./gradlew assembleRelease
adb install app/build/outputs/apk/release/app-release.apk

That makes them the right choice for sharing a build with a tester outside the Play Store, installing on an emulator in CI for end-to-end tests, or distributing internally. With EAS, setting "buildType": "apk" in an Android build profile produces one instead of the default bundle.

For Google Play itself you upload an Android App Bundle, and Play generates the APKs.

Things that surprise people:

• A debug APK may contain no JavaScript. It expects to fetch the bundle from Metro, so on a device without a Metro connection it shows a red error screen. Build a release variant to share.
• Universal APKs are large. They include native libraries for every CPU architecture. Limiting reactNativeArchitectures in gradle.properties speeds up local builds, but only ship what real devices need.
• Signatures must match to update. Installing a release APK over a debug one fails with INSTALL_FAILED_UPDATE_INCOMPATIBLE, because they are signed with different keys. Uninstall first with ADB.
• The template’s release build is signed with the debug keystore until you configure your own, which is fine for testing but never for a store.