If you’re checking if a user exists with a certain email, a backend workflow that returns a yes/no is sufficient (userExists = yes/no). It would take the user email as a parameter, and return data from API as Do a search for user where email = BE workflow’s email:first item is not empty.
Otherwise you need to manually specify how each field is used when you’re using it (you can’t reference User’s X, you have to reference the API call’s X).