Building a ISO, or customizing and ISO?

Anyone know how the Debian folks put together their ISO for installing Debian on a system? Maybe point me to a resource on how they do it? Even customizing it.

I have searched, but all that comes up is using some third party app to remix an ISO, or ways to customize the Live version.

I want to go deeper than that to be able to customize the installer, and make some packages install by default.