iPhone full

What's the difference between device storage and iCloud storage? explains "Device storage is the available storage on your iPhone, iPad, Mac, or other Apple device. iCloud storage keeps your photos, files, notes, and more, up-to-date and available across all your devices."

You cannot increase device storage capability, but If you want to free up storage space on your iPhone or iPad:

• Learn how to optimize storage space on your iPhone or iPad.
• Learn how to manage your photo and video storage.

iOS__--S21 wrote:

iCloud is a external storage system that you back up to.

That's a common misconception, and while it may provide a backup capability for your device it cannot increase your device storage. As stated "iCloud storage keeps your photos, files, notes, and more, up-to-date and available across all your devices."

But is it truly full? Sometimes it says it’s full when it’s not. Go to Settings>General>iPhone Storage and check. If it is really full, consider putting your files on iCloud by backing up, turning off iCloud backup without deleting if you can, then clearing up your device.

iCloud is a external storage system that you back up to.