The desktop environment is all the stuff like the taskbar, the settings menus, the application launcher, the login screen, that kind of thing. It’s the system level user interface.

You choose which one by which distro you download. Linux mint uses cinnamon, Ubuntu and fedora use gnome. There are “flavors” of Ubuntu and fedora that use KDE. That’s why I suggested ventoy: you can download a few different ones and boot into them without making a new thumb drive.

If you don’t feel like bothering with any of that, just use Linux mint. It’s good.