I need to add more storage to my I phone . How do I do it ?

You cannot add storage to the phone. You can increase your iCloud storage which may help in some circumstances See: About storage on your device and in iCloud - Apple Support

note the following from that link:

You can make storage space available on your iPhone, iPad, or iPod touch when you use iCloud Photos and turn on Optimize Storage. Or you can remove content from your apps—like photos, music, and Podcasts—or delete the apps that you're not using.

The internal storage of iPhone, of iPad, and of most recent Macs, cannot be upgraded.

Here, your choices are to off-load and/or to delete, or to replace with a new or a new-to-you iPhone.

Here is how to check and how to offload:

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

An iPhone 6+ is no longer supported by iOS, so a hardware upgrade is certainly a reasonable option here. And newer iPhones are substantially faster than an iPhone 6.

I took a screen shot of my iPhone storage. Then I attached that screenshot to a text that I sent myself. When I go back into storage -> Review Attachements from Messages, I find the screenshot that I texted. Please help me then if you are saying that photos and attachments in texts do not save in more than one location.

I cleared a ton of space by removing photos from messages. So you send a photo in a message. Not only does it take up data by being saved in your Photos app, it also is saved in your Messages app. Very annoying and takes up a ton of space, typically forcing people to buy iCloud storage unnecessarily.

Any photo I send through Message gets autosaved to my photo app. That's what I am saying. It also does get saved in two places. I have deleted photos from the photo app only to find copies of them saved in conversations...

A photo you send via messages is only in Photos, it does not move it to messages and then send it. Again, since there is no user accessible file management system the phone takes care of that itself. If yours is not doing that then maybe you need to get it looked at.

You don't, sorry.

You can only delete some stuff to get more space.

Regards

Giulio

No, it is not in two places on your iPhone. It is one photo and there is a pointer to it. If you decide to save it to the Photos app it adds it there and just points back to the original photo.