Starting with iOS 16.2, iPadOS 16.2 and macOS 13.1, Apple allows users to enable Advanced Data Protection. This end-to-end encryption protects the majority of iCloud data, even in the case of a data breach in the cloud. The term vast majority is not very specific, so you may ask yourself whether this includes Notebooks’ documents.
According to Apple’s specifications, data stored on iCloud Drive are end-to-end encrypted when transferred across trusted devices. More specifically, any files manually or automatically saved to iCloud Drive are encrypted, which clearly includes Notebooks‘ documents as well.
So when you use Notebooks with iCloud Drive as storage location, your documents are end-to-end encrypted when transferred across your devices.