how can i buy storage

Note there's different kinds of "storage". Click on this link to discover what's the difference between device and iCloud storage, ways to manage your space, and upgrading to iCloud+ if you need more --> What's the difference between device storage and iCloud storage? - Apple Support

Take-away:

1. Storage on a device itself is fixed and cannot be upgraded. If you want that then you have to change your present device for one with more capacity.

2. iCloud storage is on Apple's main computers. It is not a like a normal storage hard drive but by using optimization settings on your device it can help free up device storage. Lesser used items and larger file versions can be stored in ICloud and you can get access while connected to the Internet. It does not turn a 64 GB storage device into a 64 (or whatever) + 50GB/200GB/2TB device but if you have a lot of things that aren't being used much, or you don't mind only having lesser quality photos on your device, it can free up extra space so you can make the best of the GB you do have on the device itself.