Is Ubuntu an Operating System ?
Yes. Ubuntu is an operating system. Its kernel is the Linux kernel. And the GNU utils which are the essential programs. And the desktop environment — which is the visible graphical interface — is GNOME.
Ubuntu has too many desktop environment beside GNOME. For example, it has KDE plasma and known as Kubuntu.
You can use Ubuntu like you use Windows 10 or Mac OS. It is a great operating system. Ubuntu is actually my prefered OS.
You can install Google Chrome or Mozilla Firefox and surf the websites. You can install GIMP and edit some photos. You can install VS Code and write some programs and scripts. You can install Blender and design some 3D works. You can do anything other operating systems do.
Ubuntu need less CPU power to run, and need less RAM to run in comparison with Windows 10. You old computer can survive with Ubuntu OS.