Today's #FreeSoftwareAdvent is F-Droid, a Free software app store for Android. (As in, the app is FOSS, and the apps available in the default repo are FOSS.)
F-Droid also runs the default repository used by the F-Droid app, but anyone can run their own repository, and users can add that repo and install apps from there (e.g. Bitwarden does this).
Works perfectly with GrapheneOS :)