I'm not sure what you're actually asking, and anyway as I'm not a developer the moment details get technical it can quickly get beyond my own knowledge anyway. Either way, I'll provide some thoughts that maybe helpful.
Ubuntu and flavors are built from packages; Ubuntu Desktop/Server & base products are made using packages from the main repository; where as Ubuntu's community flavors (Ubuntu MATE etc) include packages from the community repository which is universe. Each flavors work is really put into these packages.
What packages get included on ISOs can be found from looking at seed files; used by the build infrastructure, so if you want to compare Ubuntu Desktop, Ubuntu Server, Ubuntu-MATE or any other flavor I'd contrast the seed files, which are used by the same build infrastructure for Ubuntu and all flavors. The difference between them is mostly just packages included.
For Ubuntu MATE's last release (25.04 or plucky) you can explore here - Index of /seeds/ubuntu-mate.plucky Exploring from that link to compare with other flavors, Ubuntu Desktop, Ubuntu Server, even other releases shouldn't be difficult.
If you pick an seed such as desktop, the most important package (at least in my opinion) will be
* ubuntu-mate-desktop # metapackage for everything here
though that isn't complete of course; ie. ubuntu-mate-guide will of course be important to some users & its these extra inclusions that make Ubuntu MATE special which I've ignored here.
As a key aim of Ubuntu is keeping the delta (difference) between Ubuntu and upstream (Debian sid) as small as possible, a lot of Ubuntu developers (who are also often Debian maintainers/developers (DM/DD)) often commit changes upstream to Debian sid which is then imported into Ubuntu development when of course imports are happening (either Ubuntu or Debian isn't in a freeze state). This applies to Ubuntu and flavors too.
If changes made by Ubuntu devs (inc. flavors) are done at the Ubuntu repository level, its done within packages that highlight it's a Ubuntu package; easily seen in the ubuntu-mate-desktop and ubuntu-mate-guide package names I've already mentioned, ie. its clear they've not been pushed upstream into Debian sid.
Hopefully something of this was useful.