Author redirect

You can’t do that with NinjaFirewall. You would probably need to setup rules at the HTTP server level (or .htaccess) to do it.

It’s an Nginx server.

Where can we find the code in NFW library that redirects the mentioned urls?

It uses the WordPress API, not rewrite rules.
You’d need to write your own ones I’m afraid.

Just to see how the code is crafted is it located in the utils.php file?

It’s in the utils.php script:
-nfw_query()
-nfw_the_author()
-nfwhook_rest_request_before_callbacks()
-nfw_authenticate()

Thanks!