How can I buy iCloud
How to buy iCloud on iPhone 6s plus
iPhone 6s Plus, iOS 14
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YOu already have the free 5GB iCloud storage (assuming you have created an iCloud account). This page tells you how to upgrade to the highere amounts:
Upgrade your iCloud storage plan - Apple Support
Note that this is a monthly subscription, not a one time purchase: and that the storage is online at iCloud, not actually on your phone. It is accessible to any device you have which is signed into that account.