Reading this, it's my understanding this ruling says Google must host rival "app store" apps in Google play store and must provide them with a copy of all apps on the playstore for them to distribute. Developers do not opt in to the new app-stores, but they can opt out.
If you mean who gets the money from app purchases etc.; The ruling also says Google play billing cannot be required,
so each app/developer can use whatever payment processor they want
Google will have to distribute rival third-party app stores within Google Play, and it must give rival third-party app stores access to the full catalog of Google Play apps, unless developers opt out individually.
Both of those are completely irrelevant tho. Nobody cares about other app stores being available in the playstore, and aurora already can install any app of the playstore.
It's very relevant. Currently I would have to tell someone of said app stores existence. But then would have to hope they could find it first. Then figure out how to sideload after that. Now I could just say. Go get fdroid etc etc etc and install stuff from there. Much simpler and easier.
Even so, google being forced this way will make app installation/updates much more convenient for non-root and oem-rom users. And ensure rhe aurora store or a future iteration of it aren't going to flag peoples accounts for google to take action against them.