Source? That sounds very backwards but also like something that ubuntu would do...
Just follow the installer log. It does the Ubuntu standard installation followed by removal of non-essential packages (office utilities, AV utilities etc.)
On the installer image is a directory casper, there is another image filesystem.squashfs, I suspect that it is simply unpacked to the hard disk during installation. System deployment is really fast this way. Also in casper directory are two more interesting files: filesystem.manifest-minimal-remove and filesystem.manifest-remove with a bunch of packages names. And of course you should have a look at the log during the installation process, all the "apt purge" things show up there.
That is no longer the case with the new default Desktop subiquity installer in 23.04. Instead of a single squashfs, there are separate squashfs files so the minimal install is faster than default in 23.04.