Hey Chris,
The default domain stuff is standard Apache behavior--and is unrelated to the alias server feature or anything in particular that Virtualmin has done.
The alias feature uses the ServerAlias directive. It just adds a single line to the new domain that you're creating--nothing complicated at all, but superior to the automatic virtual hosts that are more commonly used for this purpose because it is not pattern-based and also respects SuExec (thus significantly more secure).
Here's the doc for ServerAlias:
http://httpd.apache.org/docs/2.0/mod/core.html#serveralias