iPhones are sold with fixed storage, and while you can’t actually add more storage, you can get a wireless card reader. These are available at just about any store that sells iPhone or computer accessories and are also available on Amazon. The wireless card reader is similar to having an external hard drive that you can store your additional photos, music, videos or other data when your iPhone doesn’t have enough space.
In order to use a wireless card reader with your iPhone, connect the reader to your computer and upload whatever data you want to have stored on the reader. When you have everything you want on your reader, install the companion app that comes with the reader onto your iPhone. That’s it!