How nginx send request to php

I have some experience in PHP+nginx servers, but I've never think about it's backend. I understand that if I use php-fpm, nginx simply redirect request to specific port. But how nginx send request to simple php with all http headers and body? I'm not sure that nginx execute shell command like php index.php, so can anyone describe me (or tell me where I can found it) how it works. I'm interested not in php-fpm, but basic web server with php (or python, for example. I think they works a bit similarly).