Well, if you want to know what Gnome is supposed to look like (I mean the “default” setup) check out Fedora Workstation. Anything that looks different from that is modified. Several other Distros ship with a default Gnome desktop as well - OpenSuse Tumbleweed/Leap, Arch’s default setup, Vanilla OS, et al.
Gnome is actually one of the more difficult to modify. By default, there’s light mode, dark mode, and… that’s it. However, you can make some pretty radical changes with extensions and user themes. While it’s fairly easy to add extensions, user themes take a bit more more work to get going, and require some knowledge of CSS to make.