I understand everything from nginx virtual hosts to php-fpm user isolation to ssl certs to generating user sub-domains. My question relates to when a user wants to use his own domain he brought from for example Namecheap or Gandi.
Why does that user need to point the DNS name servers to me, assuming I'm a host? What do I have to do to make it work? Why can't the user just point their A record to my IP?