Memory

You cannot buy more storage space. The phone will always have the amount of storage it came with. You can buy space in Apple's synching service, iCloud. See this link...

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

The only way you can gain space is by deleting photos, messages, apps, etc.. Be sure to save important photos and data to your computer or an external drive. This video may help...

Internal built memory cannot be expanded. , you can buy Icloud storage and backup your files on the drive

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