> When a website prompts me to set up a passkey, depending on the device I happen to be using at the time it could end up going to Google password manager, Samsung password manager, iCloud Keychain, or Windows somethingorother.

This is the same problem we've had for 20 years with password managers, though.

The solutions are mostly the same as well (with the change to a QR + BLE based mechanism over manual transcription to go cross-device, and a protocol to support moving passwords and passkeys over to different software rather than using a plaintext password CSV export).

Many password managers also manage passkeys (and were managing TOTP before).