How do I buy a Storage

You can not buy storage on your phone however you can buy iCloud storage with packages starting at 50GB. This means you could store your photos in high resolution in the cloud, and keep low resolution photos on your phone, the quality is less but it takes up less space - The benefit is you can still access the high quality photos on iCloud without using up your phones memory.

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Buy more iCloud storage

You automatically get 5GB of free iCloud storage for your photos, videos, files, and more. If you need more iCloud storage, you can upgrade from any of your devices.

https://support.apple.com/en-us/HT201318