They don’t have to be though, a yubikey can be used as a passkey as well.
They can be, but most people will use what is built into their mobile ecosystem of choice, with built in sync/backup. Physical secure authenticators in general use will likely remain rare, but are still useful for use cases where passkeys that can be synced or migrated are not secure enough (and you want access governed with a simple physical device).
What if you use an old-fashioned desktop PC?
if the PC has Bluetooth you could use a passkey from a smartphone - besides that, if it's a Windows PC, you could use Windows Hello (with a PIN)