Android

A mobile OS based on Linux. The core is Android Open Source Project (AOSP), although most devices run a proprietary version developed by Google.

App stores

Google Play
F-Droid: FOSS app repository

⚠️ Warning
Apps in the F-Droid repository are signed by F-Droid itself, not by the original developers. This makes F-Droid another party to trust (in addition to the app developers). Since all F-Droid apps are signed with the same keys, F-Droid becomes a single point of failure: if it is compromised, all apps installed from F-Droid should be considered compromised as well. Therefore, if possible, it is preferable to use Google Play (e.g. via Aurora Store) or an alternative F-Droid repository that contains APKs signed by the original developers (e.g. IzzyOnDroid).

See this post for more info.

See also Obtainium to install apps directly from their releases pages.