Host names are handled by DNS (or other name resolution like a hostfile). Webservers listen on IPs/Sockets, but when running virtual hosts they also look at the request header for what FQDN was used to request the page.
When a web server running virtual hosts responds to a request, it looks at the request header to see if the request is from a host it knows about, then serves up the correct page. i.e. if i have a server with virtual host for initech.xyz, DNS will point it to the IP of my web server, which is listening on the default http/s ports (80/443). Based on the configuration you can turn on/off different ports that each virtual host responds to, but from an IP/port perspective, if the port is enabled, it's open.
Also DNS can have multiple IPs resolving to the same name, and vice versa.