Okay, can they block access to those keys and/or the the backups of them? Assume that my account is terminated or that it's compromised to the degree that I cannot re-claim access to it. Can I move those keys to my new device/system without the cooperation of Google/Apple/MS?
They cannot block access. The passkeys are actually stored on your devices in a Trusted Platform Module. When moved to the cloud, they are E2E encrypted, and the transferring platform has zero knowledge of your keys.
Currently, you cannot move them to other devices without the cooperation of some cloud service, or the like. At some point you'll have to trust someone to move passkeys between devices.