GNU/Linux Desktop Survival Guide
by Graham Williams
20200129 An Android app is simply an application developed to run on an Android device. Android has a very rich and comprehensive ecosystem of applications, mostly closed sourced though mostly freely available. In general apps will be installed by default from the Google Play Store.
For an open source and freedom oriented experience, though arguably less secure experience, the F-Droid repository provides a curated collection of apps.
The APK Mirror is also useful to find apps that may have disappeared from the Google play Store.
Warning Be wary when installing any application. Google maintains a secure and curated repository in the Play Store. For general users this should remain the default repository in order to avoid installing malware. Do not select, under settings, the option to allow 3rd party APKs unless you are comfortable in so doing. Also do not root your device if you can avoid it. Only do so with extreme caution, knowledge, and experience. On the other hand, F-Droid is a repository of open source Android applications and is generally considered a well curated repository.