Why is my iPhone storage full when I have plenty of iCloud storage?

iCloud is for syncing data between your devices and for making backups. It is not meant for storage.

Having iCloud storage does not increase the storage amount on your phone. That is a fixed size and cannot be increased.

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

Here are some helpful articles that can guide you on how to free up storage space on your phone.

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

Manage your photo storage - Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

You can go into your settings then general and then click storage usage and then either delete or offload apps you don’t use, you can also make sure your standard saved data you don’t use and should free up routinely is your safari online data/cache as well as apps you use that have that option within the app’s settings, you can also opt to have a higher iCloud storage plan and it should save you a lot of usage

There is an option for when your device storage is low that optimise your photos to a lower quality level than usual.

Your regular photos will be stored in iCloud.

TeraBox is a great app with unlimited storage space https://www.terabox.com/wap/activity/recipient?code=2_4399366612755