I've recently fallen in love with the Linux system and I'm wondering: is Ubuntu an OS & Linux its kernel?
Linux is not a complete operating system; it is, as you correctly described, a kernel. The default software suite since the beginning has been the GNU system, and the name "GNU/Linux" has been used for this combination. Ubuntu is based on GNU and Linux. (Linux has also been used, notably, in a number of embedded systems such as network devices and as the basis of Android, which uses an entirely different "userland" model and is not compatible with "traditional" Unix/Linux applications.)
In addition to all of the software packages, there have always been a variety of different "bundles" with installers (or manual installation), package management, configuration tools, etc. These are called distributions, and Ubuntu is a Linux (or GNU/Linux) distribution. In general, a Linux distribution comprises what most people would consider an "operating system" (technical base, user interface) as well as a wide variety of other available software packages that go beyond just an operating system.
Well Linux is an operating system, by default it comes with a kernel and a lot of tools (provided by a "distro" which is based on it). So you could build your own Linux by compiling all those libraries and programs you think that are important for you. That makes Linux very flexible. The Linux Kernel is the most basic part of it - it communicates with the hardware and is mandatory
If you do not want to put together all these small and bigger apps you could select a "Distro". A list of many distros can be found here.
So Ubuntu is one of the many distros that are around - probably one of the most used ones. Ubuntu comes as server - without any UI features - or as a desktop with an UI (Gnome) or in other desktop flavors like Xubuntu (with lightweight XFCE desktop) or Kubuntu (with an elegant QT5 Widgetkit).
So the standard Ubuntu 20.04 is a Distro that compiled the Linux Operating System with a Gnome/GTK Desktop Environment and many drivers and tools. If something is missing, you can install that software easily.
Linux runs on many different platforms, so you could get yourself a Raspberry Pi and install a special Ubuntu ARM Edition, a Debian (which is the base of Ubuntu btw) or an Arch Linux on that hardware.
Since a lot of discussion has taken place I'd like to provide an opinion from Red Hat that states that Linux is an operating system. So I get the impression that linux can be both - depending from where you look. If someone asks me, what operation system I have, I'll always answer "Linux". (Because I run different "distros" on a daily base).