The default configuration file of Nginx is in nginx/conf/nginx.conf. The path of /etc/nginx/conf.d needs to be created by yourself.
If you need to place the main configuration nginx.conf in another location, you can specify it with the -c parameter when starting nginx

sudo /opt/nginx/sbin/ngixn -c /etc/nginx/conf.d/nginx.conf

If the main configuration is still the default and you just want to see some sub-configurations under conf.d, you can use the include command in nginx.conf

include /etc/nginx/conf.d/*.conf