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They advertise a whole product which draws all this tracking data as nice graphs to developers: https://developer.apple.com/app-store-connect/analytics/

It's essentially Google Analytics, just for apps on iOS.

Where do you think those "Daily active devices" data comes from?

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Users must opt in though. It’s part of the iOS set up to allow this and you can change it any time

This setting is opt out, not opt-in. It's unclear if opting out hides the data from Apple or just from the app developers.

It is opt-in now; when setting up iOS for the first time it asks you if you'd like to enable "iPhone analytics" (OS-wide analytics for Apple) and if you accept then it asks you whether you want to share the analytical data with the app developers.

It's opt out.

No, it’s opt in when you set up your device.

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