Config file -- ServerName/ServerAlias -- domain registration

Hi.

Trying to wrap my head around something..

I can have a test config/virtual host, with the different serverAlias

As I understand it, I can do a
--psuedo config
<v..host
serverAlias   abc.foo.com
.
.
.
</vhost>

<vhost
serverAlias  qqq.foo.com
.
.
.
</vhost>

and the user could access both sites, as "subdomains"

http://abc.foo.com  <<<

is this right???

Assume at the domain registration level/process, I had an "A" record
 @   foo.com     1.2.3.4   (psuedo record)


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

Does this still get resolved to access the tgt site?

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

thanks