Think of it this way

There’s your core of the system, the kernel part. It’s the engine of the thing but basically its the package manager. This is what Ubuntu, Redhat, Arch, etc is. It’s all interchangeable in some ways and also locked into a specific place you get your packages and updates. It could be any desktop and all of the desktop environments or just a command line.

So more often than not, the core will favor a specific desktop environment. You can always install multiple environments and they’ll work but there’s some things that are suited for one desktop environment over the other. Many of the basic apps don’t work outside their environments. KDE apps don’t always work in Gnome and vis versa.

So when you download Ubuntu, your basically says give me the package manager that points to the Ubuntu repositories that will understand your version of the core and give you prepackaged software that is meant to work with Gnome.

If you go with Kubuntu you’ll get the same treatment but with the KDE desktop environment and all of its basic stuff.

But you can install KDE on Ubuntu and you can install Gnome on Kubuntu.

You can mix and match all the desktops if you want but at some point it does cause problems because the developers make different decisions and use different software that you’re package manager has to deal with.

So some distros do things different, have different configurations and package managers. I use Arch which uses pacman (package manager) to give you core software that they keep up-to-date and test but it’s limited in what it offers. So instead it has an AUR that can be accessed though many different sub package managers, like yay

I could go on but I hope this makes a little sense about the difference in distribution and desktop environments.

If you want a Windows 98 style desktop, look at KDE. It’s a lot like how Windows works