When you turn on Messages in iCloud, messages you send and receive on your iPhone are saved in iCloud. When you sign in to the same Apple Account and turn on Messages in iCloud on another device, your conversations show up and sync there too. Your entire messages history is kept up to date across those devices, even when you set up a new device. This includes messages sent or received with SMS, MMS, RCS, or iMessage.
Make sure that your iPhone is turned on and connected to a Wi-Fi or cellular network, and that you sign in to the same Apple Account on each device.
Go to the Settings app on your iPhone.
Tap your name, tap iCloud, then tap Messages.
If Messages doesn’t appear, tap See All in the Saved to iCloud section.
Turn on “Use on this iPhone” (if it’s not already turned on).
To confirm which devices receive any SMS, MMS, or RCS text messages you get on your iPhone, go to Settings, tap Apps, then tap Messages. Tap Text Message Forwarding, then turn on your other devices.
After you turn on Messages in iCloud, any messages or attachments you delete from iPhone are also deleted from your other Apple devices (iOS 11.4, iPadOS 13, macOS 10.13.5, watchOS 1, visionOS 1, or later) where Messages in iCloud is turned on. See Set up iCloud for Messages on all your devices in the iCloud User Guide.
Note: Messages in iCloud uses iCloud storage. See Use iCloud.