In the Linux world everything that isn’t content is a program. The Linux Kernel is a program. The file manager for Linux Mint is Nemo (a program). The disk management program is gnome-disk-utility. Then you have LibreOffice, Firefox, Thunderbird, and others. The rule everything is a program, includes the desktop-environment. On Linux Mint the default desktop is Cinnamon (the most popular Linux Mint), but you could have chosen the Mate version or Xfce version.
Install KDE Plasma
There use to be a KDE version of Linux Mint but Mint has stopped its development for some reason. I like the look of KDE Plasma so I’ve installed it on my laptop. There is no need to install a new distribution like Kubuntu. We’ll just add it to Linux Mint.
Go to the Software Manager and search for plasma. The first search result will be “KDE-standard”, click it, then click “install.” It will probably ask to install prerequisites give that permission. Some time will go by after which it will say the install is complete.
Log out of your current session then click on the icon next to your name. A drop-down will appear listing the desktop-environments you have installed. Choose “Plasma.” Your original Cinnamon Desktop will stall be there. You may switch between the 2 when ever you like. All of your files and applications will be available in both.
Other Desktop Environments
You can install others is you wish just to try them out but don’t get cared away. Desktop environments (sometimes called DEs) can come with a lot of prerequisite programs and it may become confusing.
Popular DEs are the aforementioned Xfce, and Mate, but there are many more. I think there are more then 1000 but many of these are orphaned projects or just seldom used.
Some other DEs are Fluxbox, LMDE, Gnome, IceWM, Openbox, and countless others. Gnome is very popular because it is used in the popular distribution Ubuntu, but the but most of the others are seldom used.