Apple documentation can sometimes be confusing.
It can not be! Apple advertises for its simplicity.
That does not sound easy.
As I understand it, this DeviceCheck API requires its own server, just like my FirebaseAuth and Database solution.
Therefore, the use of this DeviceCheck API is not as comfortable as with the Google solution.
If an Apple user wants to buy my app, then it must additionally authenticate itself with a Google account, otherwise it will not work.
And if I realize that this solution does not work, or the app is not purchased, then do it like the other apps, with a free and a paid version.