The DEs listed for a distro will be ones you can get out of the box, i.e. you install the distro and it already has the DE. However, you can then install pretty much any DE/WM on pretty much any distro. Most of the time, you'll also get a login screen where you can choose between different DEs, so you can try multiple on the same distro to see how you like them.

Most of the 'random desktops' will be window managers, there are just a few main DEs, which each have a window manager bundled in. If you take one of the separate window managers (which can be tiling, stacking, or a mix) you'll just have a bit more work to do to make it like you want, but they can have more customisation than full DEs. You can make most window managers look like pretty much any DE, but not necessarily the other way around. If you look at !unixporn@lemmy.ml, most of those are window managers. Saying they're confusing to understand and you don't want to have to customise them to make them look nice and add any separate programs you need for a full system is fair, but saying they're ugly is kinda nonsensical, since you can make them look however you like.

As for why some distros' Plasmas look different, that's just because it is itself quite customisable (from what I hear, the most customisable of the mainstream DEs). So if you install XeroLinux, you could customise it to look like stock Plasma, and vice versa.

Long story short, don't choose a distro based on their default DE or vice versa, don't disregard window managers out of hand (but do if you just want a full out-of-the-box environment), and look at different distros' customisations, as well as !unixporn@lemmy.ml and similar, to see what DEs can look like you want, but again you don't have to decide distro based on that.