There are many ways to force Nginx to use either WWW version or non-WWW version of URLs for your site.

We use following codes all the time.

Redirect non-www to WWW

Single domain

server {
        server_name example.com;
        return 301 $scheme://www.example.com$request_uri;
}

All domains

server {
        server_name "~^(?!www\.).*" ;
        return 301 $scheme://www.$host$request_uri;
}

From WWW to non-WWW

Single domain

server {
        server_name www.example.com;
        return 301 $scheme://example.com$request_uri;
}

All domains

server {
         server_name "~^www\.(.*)$" ;
         return 301 $scheme://$1$request_uri ;
}

In both cases, for other-www, we create a altogether different server { } block. IMHO, this is cleanest and optimised way to handle www to non-www and non-www to www redirection.

There are some WordPress plugins available there which can handle this at PHP-level. But for performance reason, always handle things in Nginx, that can be handled in Nginx alone! 😉