They're actually not tied to a device. They can be synced across devices.

Bitwarden, 1Password, Dashlane, Google, Apple, and Microsoft will all do passkey syncing for you.

We don't yet have a good Linux native passkey store. I believe the credentialsd project is working on a spec for that. Once that's up and running I would expect to see some options for syncing without relying on a third party service (though, you can self-host bitwarden right now). It's definitely not seamless on Linux yet though.