How much data do I have and how to purchase more?

you can see how much iPhone storage you have by going to Settings > General > About and looking at your capacity. You can see a breakdown of this storage by going to Settings > General > iPhone Storage.

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

You cannot increase the size of your iPhone storage capacity. But you can utilize iCloud storage to try and save space..

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

You can see how much iCloud storage you have by going to Settings > [your name] > iCloud

Manage your iCloud storage - Apple Support

You can purchase more iCloud storage if needed

Upgrade to iCloud+ - Apple Support

here is a video that may help with saving iPhone storage..