I can't remember precisely what is on the source ISOs, but if you are building a custom version of Debian you likely want the upstream package source plus the D

I can't remember precisely what is on the source ISOs, but if you are building a custom version of Debian you likely want the upstream package source plus the Debian changes to that (from things like dgit if the package maintainer uses it). Since these source ISOs didn't contain things like the snap packages you'd need other things anyway to start making a downstream distribution.