What could be the reason a file (app-release-unsigned.apk from Android Studio) does not open on an android phone from g-mail?

Question

The file is in the build->outputs->apk folder from a project I built in Android Studio. What I've done prior to sending that file is open Run/Debug configurations, chose the gradle project and task "assemble", then Apply, Ok, and run the project.

Answer 1

Android requires that all APKs be digitally signed with a certificate before they can be installed.

https://developer.android.com/studio/publish/app-signing.html

This has instructions for how to sign the unsigned application.

The official documentation does not say anything about whether allowing unknown sources permits unsigned applications to be installed, but this question suggests this is the case: What is the differences between `export signed` and `unsigned` .apk file?