How do I buy space ?
How do I buy storage?
iPhone 11, iOS 14
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You can't buy more storage on your phone it is a fixed size.
Here are some helpfull articles that can guide you on how to free up storage space on your phone.
How to check the storage on your iPhone, iPad, and iPod touch
https://support.apple.com/en-us/HT201656
Manage your photo and video storage
https://support.apple.com/en-us/HT205703
Clear Other storage on your iPhone or iPad
https://support.apple.com/en-us/HT211686