...Ah, it seems I misunderstood the relationship between the OS distribution and the desktop environment.
I was assuming that using Debian would be like using an older version of Windows where things look mostly the same but there are strange differences or omissions, but the underlying system has little bearing on the GUI interface. I'm betting that as long as I run the same version of KDE I wouldn't be able to tell the difference between a Debian, RHEL, or Arch Linux distribution.
You got it pretty much, yeah. Desktop environments are vaguely like Windows themes (just the GUI you interact with) and with the exception of maybe some file locations based on how it was compiled for a distro (and whatever defaults they set), will pretty much be the same across distros. You can also install multiple of them and just pick which one you want at the login screen, Arch has a good list of the various ones
here. The other thing that's going to vary is going to be the display manager, which manages what your login window looks like. Some look like ass, last I used Debian whatever one they used fell into this, others (like gnome display manager) generally look pretty good. Arch has a list of
those too. Sometimes distros get bugs up their ass and decide to make their own but that doesn't happen often, see: Canonical/Ubuntu and Unity.
You can forgo a display manager entirely as well and then launch Xorg/X server/X11 or Wayland after logging in at the command line which will start your DE. Similarly you could install a window manager instead of a DE (like i3) and just have GUI windows and some functions associated with it instead of the whole toolbox. Pretty much the order of operations is: windowing system (Xorg orWayland) -> display manager -> window manager (I think Wayland calls them compositors) -> desktop environment.
Wayland has been trying to be a replacement for Xorg for a while but there's not been a big adoption of it. Fedora recently added it as the default if you use gnome
but you'd have to be a fag to do that. I don't have much experience with Wayland since I mainly use i3 and that doesn't exist for it. Technically there's sway which is similar but last I used it dmenu (think Windows search but useful, amongst other things) didn't work right and I couldn't be bothered.