How can I purchase memory please simplify

Agree with what KB said- Inbuilt iPhone storage cannot be expanded. However, you can rent iCloud storage for backups or to store files, pictures and documents. Links provided below for your convenience.

More info about iCloud: iCloud - Apple

Plans: iCloud+ plans and pricing - Apple Support

You can’t add any more storage to your phone. If you need more space, you will need to delete something. You can upgrade your iCloud storage space if needed.

See this Apple article: Upgrade to iCloud+

Upgrade to iCloud+ - Apple Support