Relying parties request one with residentKey: "required" (or preferred) in authenticatorSelection; at sign-in they call get with an empty allowCredentials list and receive the userHandle with the assertion. Passkeys are discoverable by definition, and conditional mediation (mediation: "conditional") shows them in the browser's autofill list.
Non-discoverable credentials (the U2F model) keep the private key wrapped inside the credential ID that the server stores, so security keys can support unlimited sites but the user must type a username first. Discoverable credentials consume limited storage on security keys and can be listed and deleted there; synced passkey managers have no such limit.