How can I get more storage on my iPhone without iCloud?

You cannot upgrade local storage in an iPhone. Whatever it had the day it rolled off the assembly line at the factory is what it will have for life.

If you are using iCloud Photos, and have enough iCloud space, you can turn on "Optimize iPhone Storage" to allow your iPhone to automatically substitute lower-quality, space-saving, local copies of photos for full-size ones. (The full-size ones remain in iCloud, from where the phone can fetch them on demand.). This is automatic - you have no control over when the phone decides to swap full-size copies for space-saving ones, or vice versa.

Other than that, your options when you run out of local storage are to

• Delete or offload things from your phone until you have a reasonable amount of space; or to
• Purchase a new phone that has more local storage

You don't. You can't increase the amount of local storage in an iPhone.

It's not like a desktop where you can swap out memory modules. What you purchased is what you're stuck with.