There are a number of sites on a single server. There is a task for each of the sites to create user with limited access only to your directory. I only know one place where nginx can be assigned to the user, it is in nginx.conf. How to do it for each site I don't understand. Please help.
In principle it is possible to run multiple nginx instances on different ports, but IMHO this is nonsense. Nginx is a very efficient server and manage all your websites from one instance. With regard to users to sites, it is customary to resolve it using PHP-FPM, so this is no problem - run multiple pools, each under its own user and with a separate socket. And then tell Nginx for each virtual host to use its socket.