They could implement a virtual authenticator, emulating a TPM or security key and running entirely in software. Of course, this nullifies several advantages of FIDO.
Alternatively, and likely a much better option longer term, they'll need operating system support that provides them with APIs to establish trust between authenticators and export passkeys wrapped with keys corresponding to trusted devices, not unlike how these OSs provide APIs for integrating password managers today. A growing number of password manager vendors have joined the FIDO alliance, hopefully to help move this along. Establishing trust between devices in a Sync Fabric is also something that needs careful consideration if not a standard, since it's a huge target for phishing.
In either of these cases Passkeys from different vendors aren't interoperable.
Alternatively, and likely a much better option longer term, they'll need operating system support that provides them with APIs to establish trust between authenticators and export passkeys wrapped with keys corresponding to trusted devices, not unlike how these OSs provide APIs for integrating password managers today. A growing number of password manager vendors have joined the FIDO alliance, hopefully to help move this along. Establishing trust between devices in a Sync Fabric is also something that needs careful consideration if not a standard, since it's a huge target for phishing.
In either of these cases Passkeys from different vendors aren't interoperable.