If you click on the link I provided to some “Lunar (23.04) Screenshots”, the background I’m talking about is the first one shown in Aaron’s post.
It’s what you see when you login and/or select your session (though most of us won’t change session very often, if at all; it’s the top left corner of the screen dropdown menu)
The rotating Lubuntu animation you see during boot & shutdown is a plymouth screen I bet (it’s hiding the system boot/shutdown text messages); the lunar package is this one.