iOS backup to iCloud: how private?
Quoth Apple:

iCloud secures your information by encrypting it when it's in transit, storing it in iCloud in an encrypted format, and using secure tokens for authentication. For certain sensitive information, Apple uses end-to-end encryption. This means that only you can access your information, and only on devices where you’re signed into iCloud. No one else, not even Apple, can access end-to-end encrypted information.


In some cases, your iCloud data may be stored using third-party partners’ servers—such as Amazon Web Services or Google Cloud Platform—but these partners don’t have the keys to decrypt your data stored on their servers.


End-to-end encryption requires that you have two-factor authentication turned on for your Apple ID. Keeping your software up-to-date and using two-factor authentication are the most important things that you can do to maintain the security of your devices and data.

According to the chart in the link below, all iCloud services are encrypted in transit, and all but iCloud.com and Mail are encrypted on the server.
Two-factor authentication is required for end-to-end encryption.
https://support.apple.com/en-us/HT202303