CloudKit

https://developer.apple.com/icloud/cloudkit/

https://developer.apple.com/library/archive/documentation/DataManagement/Conceptual/CloudKitQuickStart/EnablingiCloudandConfiguringCloudKit/EnablingiCloudandConfiguringCloudKit.html#//apple_ref/doc/uid/TP40014987-CH2-SW1 https://code.tutsplus.com/tutorials/an-introduction-to-cloudkit--cms-23775

iCloud Key-Value Store

https://learnappmaking.com/icloud-key-value-store-nsubiquitouskeyvaluestore-swift/

Subscription

https://www.hackingwithswift.com/read/33/8/delivering-notifications-with-cloudkit-push-messages-ckquerysubscription

Push Notification

https://fluffy.es/push-notification-cloudkit/ https://code.tutsplus.com/tutorials/using-apples-cloudkit-for-push-notifications--cms-28125

CoreData

https://developer.apple.com/videos/play/wwdc2019/202/ https://developer.apple.com/videos/play/wwdc2020/10650/