If you turn off the iCloud Photo Library, it will remove photos from your iPhone that weren't taken on that device. If you want to turn off iCloud Photo Library but keep your Camera Roll as it is, here's how to ensure current iCloud images aren't removed during the process.
If you turn off iCloud Music Library, you cannot access Cloud-based copy of your Mac's iTunes library. In addition, you'll no longer access your Mac Matched or uploaded tracks to stream on the other devices and any saved songs to the Apple Music Catalog.
If you sign out of iCloud accidentally, Apple holds your images in the cloud for 30 days, meaning you can simply reconnect to your iCloud Photo Library to restore your images if needed.
When iCloud photos disappeared, the most likely reason is that you have logged in to a different iCloud account from other devices. In this case, you just need to: Go to Settings > [your name] > Scroll down and sign out of the current account > Log in to the right iCloud account.
Disabling iCloud Music Library will delete all cloud music on your device, including downloads from Apple Music. Copy your music library from any iPod or iPhone to your PC or directly to iTunes in just a few clicks.
(I've omitted the sender's name so as not to single them out!) Just to be clear: If you delete a photo or video from Photos in iOS or macOS and iCloud Photo Library is enabled, it will delete it everywhere, even if you have full-resolution downloads turned on with Photos for macOS.
If you designate a new library as the System Photo Library and then turn on iCloud Photos, the photos and videos in the new library will merge with those already in your iCloud Photos. Additionally, all photos and videos from iCloud will redownload to the device.
If you turn off iCloud Photo stream on the device, Photo stream, and the images in the stream folder, are removed from the device, BUT NOT FROM iCloud. If you have any other devices using iCloud photo stream, they will not be impacted. You can also delete from photo stream, just like any folder.
After you turn off iCloud Photo Library, the Photos app on your Mac can't access your iCloud library, and any edits you make to photos on your Mac won't appear on your other devices. Your library remains in iCloud and available to other devices that use iCloud Photo Library.
What happens when you turn off iCloud Photo Library on your iPhone? If you turn off the iCloud Photo Library, it will remove photos from your iPhone that weren't taken on that device.
How to disable iCloud Photo Library on only one device
Any saved songs from your Apple Music catalog. No matter whether you have 100 favorite songs or 1,000, turning iCloud Music Library off means your devices will no longer be synced.
If you switch iCloud off they will disappear from your phone. This means your contacts are trapped inside iCloud. This is great if you want to get a new Apple device — those contacts will magically transfer when you sign in.
After that, iCloud Backup would be automatically turned off in iPhone Settings. If you just turn off iCloud backup on iPhone by yourself, iCloud will stop backing up iPhone to iCloud and it would do nothing to the old iPhone backups.
If you turn off Messages in iCloud on an iPhone, iPad or iPod touch, your message history will be included in a separate iCloud backup, as we mentioned above. You can choose to turn Messages in iCloud off for just one device, or for all your devices.
If iCloud is "off" for contacts — i.e. the slider is white — and your contacts are all still on your phone, then this means your contacts are stored only on your phone. This is great — unless you break your phone. Then your contacts are gone forever.