Android - APK (Application package)

> Android

2 - applicationId

In build.gradle, applicationId is the fully qualified package name for your application that you specified during the New Project workflow.

3 - Signing

All APKs (.apk files) must be signed with a certificate whose private key is held by their developer. This certificate identifies the author of the application. The certificate does not need to be signed by a certificate authority; it is perfectly allowable, and typical, for Android applications to use self-signed certificates. The purpose of certificates in Android is to distinguish application authors.