By default, iCloud secures your information by encrypting it when it’s in transit, storing it in an encrypted format, and securing your encryption keys in Apple data centers. In addition, many Apple services use end-to-end encryption; your information is encrypted using keys derived from your devices and your device passcode, which only you know.
For the highest level of cloud data security, you can turn on Advanced Data Protection (iPadOS 16.2 or later required). It uses end-to-end encryption on more data categories such as the following:
Device backup
Messages backup
iCloud Drive
Notes
Photos
Reminders
Safari bookmarks
Siri Shortcuts
Voice Memos
Wallet passes
With Advanced Data Protection, your protected data can be decrypted only on your trusted devices, protecting your information even in the case of a data breach in the cloud. Not even Apple can access your information.
For detailed information about the requirements for using Advanced Data Protection, see the Apple Support article Apple Support article: iCloud data security overview