Although I have 25GB storage left on icloud, still my iPhone storage is full... what should I do?

Check: Settings>General>iPhone Storage...

Then update your iPhone to iOS 13.6.1 version and follow these steps:--> Clear Other storage on your iPhone or iPad - Apple Support

Regards

Giulio

iCloud storage and the storage capacity of the phone are two very different - and largely unrelated - things. Having a certain amount of space on iCloud doesn’t impact the phone’s storage. To regain storage on the phone, you are going to need to delete content or apps.

iCloud storage is independent of iPhone storage. If you need more space on your phone, you will need to delete something.

iCloud storage does not increase the storage capacity on the device.

To make more room you need to remove some files.