It has been a while since I have dug into it, but from what I remember it works roughly like this: You login to iCloud which gives your device write access to

It has been a while since I have dug into it, but from what I remember it works roughly like this: You login to iCloud which gives your device write access to iCloud storage. The device creates a private key inside the TPM, and uploads the public key. When you add a new device, one of your existing devices has to use its hardware key to sign your new devices key. It also must be signed by a second key derived from your iCloud password (so you). Apple doesn't go into the details on this, but I believe the originating TPM also signs the key with a chain of trust that goes back to Apple itself. The keychain is encrypted locally and exported to iCloud. The key used to encrypt the keychain is also encrypted using the public keys of all trusted device keys in iCloud that have both a signature from another trusted device and from you. As a user you can also just open up Keychain Access on a Mac and ask it to generate a CSV export of your entire iCloud keychain - which is how you would go about migrating to another password manager.