You don’t need to understand how data is stored to use Multisite. However, occasionally it can be helpful to know what’s going on behind the scenes. At a nuts and bolts level Multisite stores the sites in your Documents folder in a package (which is a fancy folder that looks like a file) called “Multisite for iWeb Data.mfi.”
Inside the package there is a folder called “sites” and within that is a folder with the name of each site you have managed. In each of those folders is a single “Domain” file that iWeb uses when it’s editing that site.
Often a picture is easier to understand. This snapshot is a look inside of a package that holds four separate sites.