https://www.cnbc.com/2018/05/30/messages-in-icloud-ios-11-4-sync-imessage-ipad-iphone.html

Apple released iOS 11.4 for iPads and iPhones yesterday, and it includes a new feature that makes it easier to keep iMessages in sync between your iPhone and iPad.

The feature is called "Messages in iCloud," and it relies on Apple's iCloud storage to not only back up your messages but keep an identical copy of the iMessages on your iPhone and iPad.