Want to buy more storage

Click on this link to discover how to "Upgrade your iCloud storage plan". Click here--> https://support.apple.com/HT201318

Note the difference between storage on a device and in iCloud. You cannot increase storage on a device, you would have to buy a new device with more. You can, however, use iCloud to better manage what you do have:

Manage your iCloud storage - Apple Support - including seeing how much storage you have and how to delete items stored in iCloud such as backups, photos, files, mail, messages and voice memos.

How to check the storage on your iPhone, iPad, and iPod touch - https://support.apple.com/HT201656 - "If your device is low on storage, iOS automatically optimizes its available space while installing an app, updating iOS, downloading music, recording videos, and more. To make more storage available, iOS can remove some of your items, like streamed music and videos, files in iCloud Drive, and parts of apps that aren't needed. It also removes temporary files and clears the cache on your device. iOS only removes items that can be downloaded again or that aren't needed anymore."

Support article: What's the difference between device storage and iCloud storage? - https://support.apple.com/HT206504