Hi Miguel,
I am an archivist, not a system administrator, so please be aware in advance that I'm not an expert in this particular area.
With that in mind: my understanding is that configuring a domain or a subdomain is typically something that is arranged by a domain name provider, by altering the DNS A-Record associated with your site.
For example, you deploy an AtoM site, and it has a specific IP address, but you want to use
mycoolsite.example.com as your base URL. You will typically find a domain name provider who can register the site, and will pay a fee to have the chosen domain associated with your site's IP address. Now when people enter that URL, they will be directed to your IP address and will see your site.
If you already have a domain (e.g.
example.com), and want to change to a subdomain (
mycoolsite.example.com), you can typically contact your DNS provider and request an update to the DNS A-record to add the subdomain.
Cheers,