DE is the shell, the difference between them is this:
Development rules, Development philosophies, Language made in, Purpose of creation, and the differing features.
KDE can be highly customizable, to the point you could make it look like Gnome, but it'll never run nor be Gnome. Gnome is more closed off, any customization is reliant on extensions. LXQT allows you a more windows XP look without customization to the level of KDE, but you have more modularity with certain parts of it. XFCE can have plugins to Thunar, and the desktop of xfdesktop, all panels are custimzable to look more like what you want too, but there's not that giant variety of Themes, unlike KDE.
On the terms of philosophy: You can't just yank a project to a direction you want, you will either Fork that (making another DE) or you will make your own DE to make it so you'll command the direction and philosophy. This is the most complete answer you're ever going to get without having a multiple page essay