We use Firebase occasionally for both iOS and Android-apps and this was the first time I had anything to do with this. The problem was, that the Android-app from Google Play wasn't linked properly. Firebase complained about a wrong SHA-1-hash and so I fixed this by providing the correct one from the app-signing-certificate: I went to the Google Play Developer Console and applied some black magic aka. copy and paste. After this I expected the (past) revenue from Google Play to show up in Firebase. But it didn't.
I searched the internet for an hour, but I didn't find anything — that's the reason for this post. In the end, the client did an in-app-purchase and with a delay of one day, it was there, finally! So it seems to me, that when you link an Android-app from Google Play to Firebase, only the revenue generated after linking is visible there, even though you see it in the Google Play Developer Console.
In the end, I learned a thing or two. But this whole Analytics-stuff from Google and Firebase doesn't feel right. I don't want to spy on my users. Speaking of which: In the evening I sat down and integrated Telemetry from Daniel into a toy project, just for fun. And this feels so much better. So whenever I feel the urge to see what users are doing with my apps, I know which tool I will use.