But if I write example.com, it goes to https://wwwexample.com (Without " . " between “www” and “example”), so an error pops up that such page does not exist.
I tried to delete this certificate somehow, delete directory /letsencrypt/ and directories in it (/live/example.com, /archive/ etc.).
Well, redirects are a webserver level of feature. It has nothing to do with the certificate. Also, reinstalling a server completely is almost never a good solution, as most issues on *nix are fixable without it.
As you've reinstalled your server now, your problem is also probably gone, at least until you're back the previous state of server functionality.
You're welcome to return if any Let's Encrypt related problem occurs again.
I'm pretty sure that's a cache issue in your browser. http://dorcio.com and http://www.dorcio.com only show a nginx "welcome page". And the https:// versions are down, probably because you didn't install a SSL certificate yet.
Probably, yes. But you chose nginx, probably for a reason. It will most certainly not help you with your system administrator skills, as fixing such a simple issue as a redirect can also haunt you when using Apache.
your webserver is ready. Port 80 works - www and non-www, checking test files under /.well-known/acme-challenge - there is a regular 404 - http status not found.