How to share icloud storage with other device
How to share icloud storage to another device
iPhone 12 Pro Max
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If the other device is your device and it is signed into your Apple ID, you don’t need to do anything to share your iCloud storage with that device. iCloud Storage is tied to your Apple ID, not to a specific device. If you are only using the 5GB of free storage that Apple gives you, you will probably need to more storage to accommodate both devices.
This explains how to upgrade to iCloud+.
Upgrade to iCloud+ - Apple Support