Our discussion has to do with the dependencies that any particular distribution depends on. MS Windows and Mac OS also have many of these dependencies, (it's an eye-opener to search), but theirs are proprietary not meant to be altered. But Linux being "open source" makes for marvelous variety. Each distribution has its own geneology.
Here's one for Zorin 18 XFCE:
Linux: Dev in 1991 as a personal project of Linus Torvalds, it was initially a standalone operating system kernel.
Gnu: The Linux kernel, when integrated with GNU components, became a fully functional operating system
Debian: This distribution is a "primary level" implementation of GNULinux with its own distinct design choices, policies, and tooling, combined with a huge set of additional software packages. Other primary level ("from scratch") distributions of Linux include Fedora, Arch, OpenSUSE, etc. etc.
Ubuntu: Ubuntu is built on top of Debian’s work and stays closely tied to it. In practice, Ubuntu starts from Debian’s packages, then adds its own choices, release cycle, and polish. Ubuntu often adds newer software, custom kernels, desktop tweaks, and proprietary drivers/firmware options that Debian does not enable by default. Most notably, Ubuntu specializes in LTS (long‑term support) releases aimed at ease of use for desktops and servers.
Zorin: Zorin OS is an Ubuntu‑based distribution that tracks Ubuntu’s long‑term support (LTS) releases as its base. Zorin focuses on a user‑friendly desktop with layouts that resemble Windows, custom theming, and preinstalled tools (like its Windows‑app integration and Zorin Connect) to ease migration from other systems.
Gnome: GNOME is a desktop environment (DE) which Zorin adopted and has customized to create a user-friendly experience for the desktop user, with the purpose of making it more accessible for users transitioning from Windows or macOS.
XFCE: This is an alternative desktop environment that can be easily downloaded and used instead of (not "on top" of) GNOME). Xfce was originally created as a free, Linux version of the Common Desktop Environment (CDE) used on commercial Unix systems. It started in 1996 as “XForms Common Environment,” explicitly aiming to clone CDE’s look and feel for Linux using the XForms toolkit. (The XForms toolkit was an easier, relatively small, and capable independent forms library for the X11* windowing system, and was originally used to build graphical user interfaces on Unix workstations.)
From this initial purpose, XFCE gradually evolved into a general desktop environment focused on being lightweight, modular, and standards‑compliant.
*Speaking of X11, Zorin OS allows for two types of windowing system. Wayland is the default, but X11 is a permanently selectable alternative. Many prefer it for many reasons, one of which is that In order to use the XFCE Desktop environment, one must use the X11 (Xorg) windowing stystem.