Serving Drupal from within a sub-directory of my web root
You have to edit the nginx virtual host file of your website.
You'll usually find it at /etc/nginx/sites-available/yoursitename.vhost.

Open it with write permissions, and edit root directive at the top of server block as following:

server {
    listen x.x.x.x:yyyy; #IP:port
    server_name www.example.com;
    root /var/www/html/web/drupal;


Save the file and reload nginx conf : sudo systemctl reload nginx.service