Does Key based authentication replace or rather supplement password authentication?

Question

Is SSH key based authentication something that happens to replace password authentication or does it supplement password authentication?

Answer 1

Several authentication methods can be allowed, but only one is usually¹ used – for example, if the server offers gssapi-with-mic, publickey, password and client chooses publickey, then only the key will be used for authentication and all other methods will be skipped.

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¹ In some very rare situations, two auth methods will be used for the same connection – for example, when the first connection to Bitvise WinSSHd uses publickey it will additionally ask for the password – but this is an exception to the rule.