What's unclear to me is where these packages are coming from. Two questions:
1) How does the server know where to get the packages if there's no server path or www address in the apt-get command line?
The server paths are within list of sources kept in /etc as dpkg /apt-get record. The command has its own record.
2) Do Linux distros typically come with "dormant" packages that need to be enabled with "apt-get" or are packages usually obtained from an external distribution site?
Some distros have complete set of installable packages on the installer DVD (e.g. Slackware DVD installer) and some don't have (e.g. Mint CD) this latter relies merely on the internet connection to download and install whatever you apt-get in the command line.
You shall have better and more complete answers from here
Hope that helps.