The Server Blocks are for setting up virtual hosts. This will allow nginx to serve different web pages for different domain names that all point to the same server allowing multiple domain names and web pages to be served from the same single droplet.
You don’t have multiple domain names or even one domain name, so you had no need to go and change the server_name inside your server block you should have left it alone.
Because you don’t have a domain name your only way of accessing the web pages on your server through a browser on the internet is by entering your servers IP into the address bar. The default server block is what handles these requests.
The default server block means the default web site that will be served when a HTTP request is made to the server by a user that doesn’t specify any other pre-defined vhosts/domain names that exists within your server blocks.
So the default server block will handle and process requests made to the server that are made by entering a raw IP address just like you have to because you don’t have any domain names.
You can’t use bbb15.it as your server name because it isn’t a registered domain name and has no public DNS records pointing to your server. You’re not going to be able to enter bbb15.it into your address bar in a browser and expect your server to serve it because there’s no DNS for it and it doesn’t exist on the public internet.
You COULD access your website through http://bbb15.it (which is how you have configured it currently) ONLY if you was browsing locally from within your servers network, you won’t be able to access that from outside your servers network.
You should have just left your default server block as it was and changed the document root to /var/www/bbb15.it/html and then you could access your web site through your servers IP.
You said you wanted to access it like this http://ip_address/bbb15.it in which case you then need to enter /var/www/bbb15.it/html from your shell and mkdir bbb15.ir and place your web pages inside that folder.
I think you should go back to the basics and have a read up on basic networking as this is very basic stuff. I would suggest going to google and have a read up on the difference between the “intranet and the Internet” this should give you an overview and basic understanding of what I am talking about.
Best of luck