How do I get more storage

The only way you can gain more storage on your phone is by deleting photos, messages, apps, etc.. Be sure to save important photos and data to your computer or an external drive. Check this link...

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

This video may be helpful...

You cannot extend the storage capacity of your iPhone. The total storage capacity of your iPhone will forever remain the same as it was the day you bought it.

Axel F.