Obtainium takes a bit to learn, mostly about what URLs will be understood as a source.

GitHub and GitLab repos with releases will work, for example with Amethyst:

https://github.com/vitorpamplona/amethyst

You can use F-Driod URLs:

https://f-droid.org/packages/com.termux

And then others like Signal with just:

https://signal.org

There isn't a directory to search for apps with Obtainium. So it is necessary to use search engines, repositories like F-Driod and etc to find apps.

You can install from pretty much anywhere, except Google Play (it requires a login, not sure how Aurora Store does it).

The GrapheneOS App Store is used only for the apps that come with the OS and other system updates.