I was curious about this and found this question here: What is the difference between installing Kubuntu and Gnome with the Plasma Desktop
But I don't have enough rep to ask a further clarifying question. I also think my question's phrasing is a little more precise to what I'm actually trying to understand as it doesn't contain as much confusion over the relationship between KDE Plasma and Ubuntu etc.
It seems to me that if Ubuntu is just the OS with Gnome installed by default, and Kubuntu is just Ubuntu with KDE installed by default then installing KDE on Ubuntu or installing Gnome on Kubuntu should basically amount to the same thing right?
I asked around on Reddit and the answer I was given is that they are basically the same except for some of the ways the specific applications that make up the DEs are configured, so with Ubuntu there are specific configurations that ship with it's Gnome DE that might not be configured that way if you just installed it on top.