The data stored on iCloud is locked with the key from the devices’ Secure Enclave. They’d have to unlock your device to get access to decrypt the iCloud data.

I already linked to this article on Advanced Data Protection for iCloud (e2ee for most things) in a different comment, but it feels like a lot of people don’t know about this feature. It literally has zero effect on the user experience (except janky access to iCloud via the web, but shrug). Apple’s competitors don’t have anything close and their business models mean they probably never will.

https://support.apple.com/en-ca/102651