DND_Reborn
The High Aldwin
Interesting. I'm glad you chimed in again.Didn't think this would go on for 5 pages, but it looks like my original question has led to a couple tangents. Welcome to the Internet!
Anyway, I'm going to ignore all those and go back to my original ask of "How would you describe it to players?"
And I'm going to share how I handled it at my table since it became relevant in a complex scenario. You all have discussed a lot of this already. I'm just compressing it as best I can. I added "IMO" clarity for my table.
I worked from the premise that there are two items in contention: the environment ("objective") and individual perception ("subjective").
WRT lighting, areas of the environment can be lit, dimly lit, or dark.
Time of day, location, spells and objects affect lighting.
Perception doesn't affect lighting, only the viewer. DS and DV are perception affecting abilities.
IMO, a creature with 120' of DS and DV sees as if their environment were lit, even if it is dark or dimly lit. But because several abilities are dependent on recognizing lighting differences, they can continue to be aware of those differences. So they know when an area is dark or dimly lit, but they can see as if the area was lit anyway.
Because naughty word magic.
The main reason I had to rule this way is because they players ended up in an area constructed of shadows, and housing shadow creatures. I had to consider whether shadow was dark or dimly lit. It's certainly not a lit area. But then logically, shadows (and therefore all creatures and objects in this realm) ran the risk of being partially or completely invisible to creatures with DS and DV.
In the end, for this situation, I had to give shadow substance. Everything was shades of gray, but it was all real. A shadow blocked vision (which did confuse the DS/DV character for a while, because it was darkness that couldn't be seen through).
And so we come back "normal" environments. My ruling would extend here: shadows have substance. Much like the external visual impact of fog, rain, or snow, a shadow is an environmental (para-elemental) effect that persists beyond the concepts of dim light and darkness.
Light and dark may be opposites of a spectrum that includes dim light. But for my game, shadow is not in that spectrum. The interaction of light and dark can produce not only varying levels of light, but also shadow, which is a different thing entirely.
And the first impact of this (that just popped into my head), which I would allow at my table, is that a character that made a successful stealth roll to hide in shadows would remain hidden even if they were in the field of vision of a creature with DS/DV. Perception does not change the environmental reality. All bets are off if the environment changes (frex, a Light spell that gets rid of the shadow).
YMMV.

If someone is hidden in shadows, they are using the shadows created by light to hide themselves. In effect, they are still in light, even if dim.
Now, this is different from someone standing out in the open in darkness, beyond the edge of light. This person is not hidden in shadow, but in darkness. DS allows sight in darkness as if bright, so then the hidden person would be obvious, just standing there out in the open.
Anyway, its late so I hope that made sense. If not, I'll revisit it in the morning.