Re: Config file -- ServerName/ServerAlias -- domain registration

> but what happens if a user inserted the  "www" at the front?
> http://www.abc.foo.com  <<<

If there's no matching ServerAlias or ServerName for it, it goes to
the "default" (first-listed) virtualhost for the IP:PORT.
apachectl -S shows you a summary including which one is the default.

> Is there something that has to be done at the domain registrar level to handle this for the dns resolution?

Anywhere you will use a hostname beyond stuffing it in your own hosts
file, then in practice yes it needs to exist in DNS.
The DNS hosting may not necessarily be your registrar.