Let’s start with a smartphone. A user creates an account with a passkey for a service, that passkey gets stored on their smartphone, and they can use biometrics to sign in from then on. The private key is stored on the smartphone. Great.
But then how do you sign into that same service from a different device?
If it’s by using a password manager, some third party piece of software, How do you sign in on a device where you’re not allowed to install third party software?
Passkeys should display a QR Code for you to scan with a trusted device if you try to use them on a device that doesn’t have your passkey stored.