SSH key based login is not vulenerable to MiTM attack. Is it true?

SSH key based login is not vulenerable to MiTM attack. Is it true?

I read somewhere that when using key based login instead of a password based login, it is not possible for a Man-in-the-Middle attack to happen.

This question is not about what is being more secure: key based login or password based login.
Instead I am interested in knowing:

1. the steps taking place during a key based login,
2. why it is not susceptible to MiTM attacks.