iCloud storage

I see you are asking about turning OFF iCloud, not turning on.

You can do that but in order to have your device backed up you will need to do so to a computer manually. In addition, if you have content on other devices it will not transfer automatically (sync) with your iPhone.

Here's informative articles:

Change which apps sync and store data with iCloud - Apple Support -

"When you turn on iCloud for an app, its information is safely stored in iCloud and automatically kept in sync across your devices. When you turn it off, the app will no longer connect with iCloud, so your data will exist only on your device. You can choose which apps on your device you'd like to use iCloud, or turn off iCloud completely."

When you turn off an iCloud feature on a device, information stored only in iCloud for that feature is no longer available on the device. If you turn off Mail, Contacts, Calendars, Notes, Reminders, or iCloud Photos on just one device, you can still view and change your information on iCloud.com and on another device you set up for that feature.

Sign in to iCloud on all your devices - Apple Support

How iCloud keeps information up to date across all your devices - Apple Support

This article explains how to use iCloud on an iPhone

Use iCloud on iPhone - Apple Support

If you turn on iCloud, you get 5 GB of free iCloud storage. Information such as contacts, calendars, bookmarks, notes, reminders, voice memos are stored in iCloud.

You must use the same Apple ID.

See -> Use iCloud on iPhone -> Use iCloud on iPhone – Apple Support (UK)

If you turn Off you don't get the 5 GB free.

As posted:

If you turn on iCloud, you get 5 GB of free iCloud storage. Information such as contacts, calendars, bookmarks, notes, reminders, voice memos are stored in iCloud.

You must use the same Apple ID.

See -> Use iCloud on iPhone -> Use iCloud on iPhone – Apple Support (UK)

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And you can't back up your iPhone, iPad, and iPod touch with iCloud.

See -> How to back up your iPhone, iPad, and iPod touch with iCloud

How to back up your iPhone or iPad with iCloud - Apple Support

You need to back up to your computer.

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Read Apple article -> what is iCloud -> iCloud+ - Apple