this might not be very helpful but graphene does not choose what to use to install apps.
you can use just one source or two, just keep in mind using more methods to install apps will increase the attack surface.
What method you use to install apps will depend on your thread model and preference.
So what I am using is GooglePlaystore for most apps (I need the Playstore for my banking apps) and Obtanium to install the Fossify apps as they where not on the Playstore yet when they forked from smt and did their first app release.
Using the Google Playstore for installing apps is one of the most secure methods. There a lots of things Google does that are not great, but they are very very good when it comes to security.
The Aurora Store is a front for the Google Playstore that allows you to download apps without google account. I dont use it myself as
F-Droid is an Appstore like the playstore but is focused on Open Source apps. I dont remember all the details on top of my head but I know that there are some problems with their security. I often see the recommendation when using F-Droid to use the droidify app instead of F-Droids own app.
Obtanium: an app that for installing apps and keeping them updated when downloading the app from a website such as github or directly from a developers website.