GOS-user Google Play products coming from Graphene's mirror version of it aka Google Play Services app
Not sure if I'm understanding this part correctly or not, but the version of Google Play that you can install from the Apps app is the real Google Play. After setting it up correctly, Google Play will update itself and Google Play Services.
So, what you'd be using would not be considered a "mirror."
GOS-user Will Google know I have an app from them??
yes
GOS-user Do banking apps actually WORK when downloaded that way?
Maybe. It's best you read this section of the GrapheneOS website: https://grapheneos.org/usage#banking-apps