[Infernal]

> Or will the iPhone have a multi-hour update where it decrypts its entire iCloud archive on the client-side, and then reuploads it without encryption?

More likely that the phone just sends the keys to Apple in that case

[chatmasta]

The phone doesn’t have (all of) the keys. That’s the point. I had to save a passphrase somewhere out of band.

[Infernal]

But that passphrase you saved is an additional key, in case you lose all your Apple devices for example. You can tell it isn’t required for your phone to decrypt data because you don’t have to type it in to access your data, or even migrate to a new phone.