Managing Storage on iPhone
When you set up iCloud, you automatically get 5GB of storage. You can use that space for your iCloud backups, to keep your photos and videos stored in iCloud Photos, to keep your documents up to date in iCloud Drive, and more. You can always check how much iCloud storage you're using in Settings on your device or on iCloud.com. If you're running low, you can manage your storage to make more space or upgrade to iCloud+ any time.
Not sure about the difference between the storage capacity that comes with your device and iCloud storage? Learn more about device storage and iCloud storage
Make more space available in iCloud
If you run out of iCloud storage, your device won’t back up to iCloud, new photos and videos won't upload to iCloud Photos, and iCloud Drive and other iCloud apps won't stay up to date across your devices. And you also won't be able to send or receive emails with your iCloud email address.
You can free up storage in iCloud by deleting content that you're not using. For more information about this, please visit the link below.