I admit to newbie-style confusion but will ask this anyways;
I read that
So, is the ‘expanding universe’ due to physical movement within existing space, the growth of space itself, some combination of the two, or something else?
The very short answer is that space-time (you can say space, but it’s really shorthand for space-time) itself expanded, dragging everything else along with it.
That expansion of space-time continues. There are parts of the universe we can’t see. In the parts we can see, things are moving apart within space, except where local gravity overrides the more general expansion. There seems to be a force, called dark energy because nobody knows yet what it is, that is pushing the expansion faster than should be expected from the known initial conditions but not much can be said about it.
The long answer gets technical.