How can I buy more iPhone storage

iCloud virtual storage and iPhone physical storage are two completely different things and one cannot be substituted for the other. The iPhone physical storage is set when you bought the iPhone and cannot be increased. Perhaps these articles will help:

What's the difference between device storage and iCloud storage? - Apple Support

Manage your photo and video storage - Apple Support

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

iPhone storage can never be expanded beyond what the phone came with when you purchased it. Buying more iCloud storage does not increase the physical space on your iPhone. iCloud permits you to sync more data with iCloud if you run out of space on the cloud by needing more space than the free 5GB of iCloud space.

If you are out of space on your phone, you will have to delete things you don't need or use, such as Apps, Photos, Music, Videos, Files, Messages, Email, etc. Or perhaps it's time to buy a newer iPhone with more physical memory.

You cannot buy more storage space on your phone. It will always have the same amount it came with. You can buy space in Apple's synching service iCloud. The only way to create space on the phone is to delete photos, messages, apps., etc.. Be sure to save important photos and data to your computer or an external drive. Check these links...

What's the difference between device storage and iCloud storage? - Apple Support

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

Be sure to restart your phone after deleting...