I found a work-around: Reading "/var/log/nginx/error.log" includes a warning that Nginx can't find "/usr/share/nginx/html/favicon.ico", so for some reason, Nginx uses /usr/share/nginx/html/ instead of /var/www.

I'm confused about the multiple configuration files used by Nginx:

/etc/nginx.conf
/etc/conf.d/
/etc/sites-available/
/etc/sites-enabled/

Why are there more than one?

Thank you.