How do I add more storage to my phone?

The link below has the general information you need about different kinds of "storage", how to get more, or how to better manage what you have. Storage on a device is fixed and cannot be changed other than by buying a different device. You can use iCloud storage to free up space on a device, as outlined in the above article. You get 5 GB for free but if you're in serious need of more storage then there are subscription upgrades.

What's the difference between device storage and iCloud storage? - "Learn the difference between your device and iCloud storage, ways to manage your space" and upgrading to iCloud+ if you need more. Click here --> What's the difference between device storage and iCloud storage? - Apple Support

The support article 'How to check the storage on your iPhone, iPad, and iPod touch' also describes how optimize the storage on your device by interfacing with iCloud storage. - How to check the storage on your iPhone and iPad - Apple Support

You don't.

Storage on the iPhone is fixed. It cannot be increased.

If you need more space on your current phone, you need to start deleting apps, video, music, messages, photos, etc. to make more room.

If you decide to buy a new phone, buy more storage than you think you will ever need. Things always have a way of filling up much faster than expected.

You sell that phone and purchase a new one with more storage. You cannot update an iPhone storage once it is purchased.