Certbot doesn't seem to autorenew on my Debian 9.8 installation

I am struggling to understand the difference between sudo certbot --nginx certonly and sudo certbot -i nginx

I also notice your command seems to only address renewal sudo certbot renew -i nginx --cert-name example.com

Does sudo certbot renew -i nginx --cert-name example.com make any changes to my nginx configuration? (I would like to understand what I am doing when I run that.)