Open source apps in Android: where to find them
There are several Play Store alternatives to install open source apps in Android, most include the main open source app repository: F-Droid.
F-Droid is a FOSS (Free and Open Source) Android app repository. It has hundreds of open source applications for your device. F-Droid also has its own app. You can search, install and update apps like in the Play Store.
There are other great app stores that include F-Droid repository, like Foxy Droid. This store is lighter than F-Droid (its APK only weights 1MB) and works well in old devices.
Although the main FOSS repository is F-Droid, there are other repositories like the Guardian Project repository (which includes Tor-related apps). You can activate this repo or add others both in F-Droid and Foxy Droid.