Depending on the site, you can use one device to login to another without installing additional software. For instance, if you have an iPhone with a passkey for microsoft.com stored on it, you can login to Microsoft.com using the iPhone.

Here is a webpage that has some screenshots to show you what I mean. You can probably google some other examples.

appleinsider.com/…/microsoft-finally-lets-users-s…

It is possible to sync passkeys across devices but at this point is mainly within a single ecosystem.