iCloud backups are 100% encrypted. Only some iTunes backups are not, and only if the option to use encryption to protect the backup is not selected.
They are, but apple have the keys : https://thehackernews.com/2016/01/apple-icloud-imessages.htm...
So basically, they could be in clear text, it's pretty much the same.
Please see this link[1], Apple explains exactly how keys are stored in their datacenters (Hint: it is not in clear text). They use HSM's which destroy the user's key after 10 failed attempts.
[1]: https://www.apple.com/business/docs/iOS_Security_Guide.pdf
EDIT: backups are encrypted, but apple have the keys. See below.