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iCloud storage is on the server and is accessible to all devices signed in to the account. It is not storage on the device itself - this is not upgradeable. And I don't know where you got $12 from - the 50GB storage level is $0.99 per month (for the account, not per device). Higher levels are available: 200GB is $2.99 per month, 2TB is $9.99 per month.
Question: iCloud storage