This post is from 2011, about 14 years ago. Some of it may be out of date.
▸ Syncing Your App's Preferences with iCloud
Jake MarshOne of the most interesting additions to iOS 5 was iCloud storage which makes it easy to sync data between apps running on different iOS and OS X devices. Effectively you can store key application data in the cloud and have it propagate to all instances of the app wherever they are running.
Nice writeup about syncing NSUserDefaults data to iCloud using NSUbiquitousKeyValueStore.
Link: useyourloaf.com/blog/2011/10/24/sync-preference-data-with-icloud.html