Samples have HKDevice associated with them and you can determine what the local device is. More information available here: https://developer.apple.com/documentation/healthkit/hkdevice/local() You can also utilize this in predicates for queries.

The HealthKit datastore is inaccessible when the device is locked, so you will not get any updates at that time.

HKElectrocardiogramQuery is only available on iOS14.0 (https://developer.apple.com/documentation/healthkit/hkelectrocardiogramquery?language=objc) So its usage on an iOS 12.0 device will cause crashes. If you want to support iOS 12.0 devices, you'd have to set the minimum iOS version in the build settings under iOS Deployment Target. And if your app uses API that were introduced in iOS versions > 12.0 then you need to add availability checks in your code.