netstat show that nginx is listening on port 80 but there is no record of it listening on port 443.

Right after installing and configuring nginx (the other day) I followed the instructions in Let’s Encrypt (the newer version for ubuntu 18.04) and supposedly I have the SSL certificate. (I’m not sure how to verify that.)

Since I am clueless about how UFW really works, I don’t understand how UFW and the certificate which is installed in nginx are connected.

You need to make sure that you have a valid HTTPS server block, otherwise your Nginx would not listen on port 443 as you saw with the netstat command.

Here is my present server block:

server {
listen 80 defaultserver;
listen [::]:80 defaultserver;
listen 443 ssl defaultserver;
listen [::]:443 ssl defaultserver;

    include snippets/snakeoil.conf;

    root /var/www/html;

    index index.html index.htm index.nginx-debian.html;

    server_name musingsofamaverick.org;

    location / {
            # First attempt to serve request as file, then
            # as directory, then fall back to displaying a 404.
            try_files $uri $uri/ =404;
    }

}

The snippet snakeoil.conf contains the following:

sslcertificate /etc/ssl/certs/ssl-cert-snakeoil.pem;
sslcertificate_key /etc/ssl/private/ssl-cert-snakeoil.key;

Not sure how to validatew this server block.

You can validate your config syntax by running:

nginx -t