What is APK File? How does it work?
APK is the acronym for "Android Package", which is a package file format most widely used by the Android operating system for the distribution and installation of mobile apps and middleware.
How to install unknown sources Apps APK file to Android?
Out of the box, your Android device only allows you to install apps from Google Play Store. To install software anywhere else, you have to enable installation from third-party sources. That is, enable the "Unknown Sources" setting.