I have a much better understanding of how that works now, thank you Olivier. My compliments to Jon, once he overcame his surprise at how little I knew, he was kind enough to point me in the right direction.

As I am a relative newcomer to Linux administration, and most power users suffer from the curse of knowledge, occasionally I run across issues that are so trivial to the experts that it's difficult for me to find help for them. In case it would help someone else in the future, I thought I would post this particular one here.

I use both Borzel's excellent XCP-ng Center as well as XOA to administer my systems, but in this case was using XOA. What I needed to do was increase the size of the disk being used by a VM running Ubuntu from 4GB to 24GB. Easy, right? I just change that in XOA and I'm done. Of course, that wasn't all there was to it, as the VM doesn't recognize the additional space.

Thinking (in ignorance) that there was a problem with my XOA, I chatted with Jon in support. I'm sure he must be convinced that I'm an idiot, but he was kind enough to explain to me that there were additional steps involved to get the allocated space to be recognized by the VM. He suggested a couple of places to read up on this that didn't end up having what I needed, but I eventually found the correct procedure here:

https://www.rootusers.com/lvm-resize-how-to-increase-an-lvm-partition/

Hopefully that will be of assistance to someone else in the future.