Slackware, Ubuntu, Mint , Debian... They're called distributions and they're well... Distrubutors of software. Imagine they're like different app stores.

Not all software is present in all distros, but the most used ones are. And for everything else, there's Flatpak, which is sort of a distro-agnostic app store.

So there are several desktop environments (DE) out there, the big ones being GNOME and KDE. And distros will usually pick one as the default but will also offer others.

As an example, Debian doesn't lean much on its default DE, you're very much free to pick whatever you want. Mint, on the other hand, will only truly support Cinnamon, even though it packages other DEs.

Another thing is that not all software will be at its latest version on all distros. The stable branch of Debian is favored by server usage because they take a very deliberate approach to updates, making sure what they distribute is rock solid before shipping. The side effect is that packages are often quite a few versions behind the bleeding edge. Rolling releases like OpenSUSE Tumbleweed are always updating, so you're close to the latest software, but also closer to bugs.

Hope I helped.