I had a 2016 SE (same camera as her 6S) and I now have a 2020 SE. I find the camera a definite improvement:
--In really good light (bright sunshine, or evenly-lit indirect light, I don't notice any difference between the two.
--In dimmer light I notice more definition on the new camera. In situations where the light is good but there are portions of the photo that are in somewhat dimmer light, then I notice that the new SE camera brings better detail out in those dimmer areas than the old SE camera did.
--In darker photos, neither camera is great (you need the more advanced systems with Night Mode to truly make a difference), but I find the new SE brings out more detail.
--Though I haven't played around with it much, the new SE camera has Portrait Mode, the old one doesn't.
Overall I like the color better on the new SE than the old (though that is a personal preference of course). The new SE has a wider angle lens: from the same position shooting the same thing, there is more in the frame on the new camera.
You know what? I just re-read your post and you wanted a comparison to the XR, not to her 6S, and I misread that. Still, I'm going to leave the post as-is and you might find it useful.