My phone storage is full but I have room in my iCloud, why?

Hello ~ Take a look here as it explains well…

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

The link above explains the difference between the two.

~Katana-San~

Because iCloud storage has nothing to do with the storage on your iPhone.

If your phone storage is full, you need to delete stuff from your iPhone to free up storage.