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?

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.

Don't they verify executables for this reason? Do they provide info about whether they log this process?

I believe iOS handles this process differently. Apps are signed in advance (when the app is approved and published to the Store) and the signatures are only checked locally against a hardcoded signing key. But even on Mac when it comes to notarization, I'm pretty sure the signatures are only checked on first run and then the result of that is cached (partly for performance reasons).

Same on Mac AppStore.

Why would they verify it everytime you run the app? It already goes through heavy review before it comes onto the app store.