Apple Resources - iCloud & iCloud Drive

Introduction to iCloud

  iCloud is the service from Apple that securely stores your
  contacts, photos, files, notes, passwords, and other data in
  the cloud and keeps it up2date across all your devices,
  automatically. iCloud also makes it easy to share photos,
  files, notes, and more with friends and family. You can also
  back up your iPhone, iPad, or iPod touch using iCloud.

  iCloud includes a free email account and 5 GB of free
  storage for your data. For more storage and additional
  features, you can subscribe to iCloud+.
iCloud (is built into every Apple device)
iCloud User Guide

What does iCloud back up?

iCloud Drive (Any type of file - All in one place)   

  The files you keep safe in iCloud Drive are easy to get to,
  no matter which device you're using. You can access all your
  files from the Files app on iOS and iPadOS, the Finder on
  your Mac, or

How does iCloud work? (advanced)   

iCloud Security (Data categories and encryption)
E2EE (End-to-end encrypted data)
  End-to-end encryption provides the highest level of data
  security. On each of your devices, the data that you store
  in iCloud and that's associated with your Apple ID is
  protected with a key derived from information unique to that
  device, combined with your device passcode which only you
  know. No one else, not even Apple, can access end-to-end
  encrypted information.