Linux

Each distro picks the things it likes the things it doesn't like and it combines what it wants into a working operating system. Maybe they make some of their own custom stuff, maybe they just borrow other people's stuff.

Debian, up until the last couple of revisions was very big choosing on only free things. If you wanted to use any non-free products you had to jump through small hoops. So Ubuntu took debians core, and rewired it to properly support free things making installation and maintenance on newer hardware much easier. Because it was so much easier, they got a huge support community, and became the default for a lot of people just starting out. But then the guys that run Debian also made other decisions, like trying to monetize some of the aspects or push for the use of different package managers that people don't love. Mint came along and kind of filled the gap in between ubuntu's up sides and downsides and became the easy default for a lot of people. In the midst of all that turmoil, Debian slipped in their own version of making non-free software seamless. A lot of the support thrown into Ubuntu and mint also helps Debian.

Red hat, fedora, and centos have the same kind of story going on, But it's much less exciting and more about support and payments.

The next thing you weren't into is immutable operating systems. Like Fedora silver blue or NIXOS. They're extremely secure, because you're not allowed to make changes to the operating system blindly while it's running, But it complicates just about everything you do in the name of security.

The other thing you mentioned were window managers. (Gnome, KDE...) They're basically affecting the look and feel of the gui for the operating system. It's your right click and your start menu and your window shades at the top and how windows are moved and snapped and organized. KDE looks and works by default a lot more like Windows, Gnome has a rather flexible plug-in system in tons of plugins available. Most of the other window managers are designed for low memory usage.

Another thing you'll run into is X-Windows and Wayland. They mainly deal with backend internals of how the gui does its work. X Windows is ancient and compatible with just about anything that was ever made, Wayland is a bit flashier a little more efficient, and a little more secure, But at the same time it has a lot of compatibility issues with new hardware. Like if you're going to run auto hotkey you're going to have a harder time getting it to run under Wayland.

If you're running on an x86/64 PC you can choose whatever you want, with the lion share of tech support being available for Debian variants (Debian, Ubuntu, Mint)

If you're running on Mac, some distros are better supported.

If you're running on a raspberry pi you're usually best going with one of the ones they recommend.

When someone says that an application is tested to work with a certain distribution, most likely it can work or be coerced to work with most of the other distributions, But the developer designed it under and tests it under whatever distribution they recommended regularly. So don't be surprised if you choose something else and you have to fight with it a bit to get it to work or in rare cases it doesn't work at all.