nginx rewriting sub domain into a directory internally

To make a long story short, the most efficient way of doing this is simply to change the document root according to the sub domain

So within your server stanza, you can simply do the following

server_name ~^(?.+)\.tech-g\.com$;
    root /var/www_nginx/$sub;

This way, when you add a directory such as myothersite.com inside your nginx web directory, visitors comming to myothersite.com.tech-g.com will end up being served content from the directory /var/www_nginx/myothersite.com

No rewriting URLs needed, and no evil if statements either