Google Play Install Referrer
The Google Play Install Referrer is an Android API that gives an app the referrer attached to the Play Store link it was installed from, so the install can be matched to the click exactly.
When a link sends a user to Google Play with a referrer, the app can read it after the install through the Install Referrer API. It's the strongest attribution signal on Android: deterministic, free, and requiring no device fingerprinting.
It only exists for installs that came through Google Play — a build installed from Android Studio or with adb install has none, which is why deferred deep links should be tested through a Play testing track. iOS has no equivalent.