Passkeys _may_ be synced, but that isn't guaranteed. For example a "device bound passkey" isn't synced.
There is a project under way to specify how to "sync" device-bound keys between authenticators: https://fidoalliance.org/specs/cx/cxp-v1.0-wd-20241003.html
Ideally this should have been hashed out before deploying passkeys everywhere, but I guess you can always register multiple passkeys for the sites that allow you to.
alt Hacker News
There is a project under way to specify how to "sync" device-bound keys between authenticators: https://fidoalliance.org/specs/cx/cxp-v1.0-wd-20241003.html
Ideally this should have been hashed out before deploying passkeys everywhere, but I guess you can always register multiple passkeys for the sites that allow you to.