The core of Ubuntu is built on Debian, but the maintainers create a lot of their own packages based on the latest versions of software available. So for example both distributions have a version of Python available, but the one in Ubuntu might be the very latest release while the one in Debian may be several months old because it goes through more testing. I like to compare their usage to the difference between using Ubuntu for a desktop computer (where it’s not the end of the world if something breaks) to using Debian for a server (where you absolutely need rock-solid services that will never fail you). And of course Ubuntu and Debian are not your only choices, there are even other distributions which are based on Debian or Ubuntu, several distributions based on Redhat, and plenty of others which are completely built in other ways. If you really want to learn the nuts and bolts of it, check into the Linux-from-scratch project which walks you through building your own system completely from compiled source code.

Now the desktop environment is a whole different thing, and Linux gives you a lot of choices there too, from very lightweight desktops that could run on a twenty year old computer to much heavier desktops with a ton on features which require more modern hardware. The nice thing about desktop environments is you’re not stuck with just one. You can actually load up several and choose which one to load when you log in to the desktop, then later on dump the ones you don’t like. So the important thing is finding a base OS you like first, then everything on top of that can be changed as you learn more about Linux.