If you're a new user and you're trying to add a source:
Sources are not added via global sources, OBS uses right-clicks to add/remove/etc, so you right-click the sources box to add sources.
Global sources are usually for devices. When you add a device to the global source, you then add "instances" of that global source to your scenes after doing so, and that global source will "stay on" throughout all your scenes, so it's used to prevent devices from loading/unloading every time you switch scenes, that's all.
