How can i buy this storage should i pay every month or just one time pay

You can “rent” additional iCloud storage. This does not add any storage to your phone, which will never have more storage than it did when you bought it. iCloud storage can be used for backing up your phone, and also for syncing data to other devices that share your Apple ID. iCloud also is not an archive where you can offload data to free up space on your phone (with a couple of exceptions). Here’s a description of the differences between iPhone data and iCloud data→What's the difference between device storage and iCloud storage?

Here is more information on upgrading your iCloud storage→Upgrade to iCloud+ - Apple Support

Note that if you upgrade to any level of iCloud+ it also comes with iCloud+ Private Relay, which is sort of like a super VPN that protects your identity when accessing websites.

You'll be charged each month until you cancel it or a problem occurs with the payment method.

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