I am confused by lowlight performance as in more detail in lower light conditions from camera to camera or sensor to sensor
My Nikon D-500 can get to ISO 50-1,640,000 (Boosted) according to the DPReview, while the Nikon Z8/9 is ISO 32-102,400 and is said to have better low light performance. I thought the higher the ISO setting the better photo in lower light condition, And I confused? It sound like the D-500 would be better.. OR is it the sensor design of the newer cameras that make it better in lowlight. If I read correctly the D-500 and the Z-9 with the same settings in the same light conditions the Z-9 will have less noise and crisper images.
Some things to unpack in your post.
The available high ISO limit itself, much less any extended high ISO settings doesn't really determine the quality of images in low light conditions. The noise performance and dynamic range of the sensor/camera at specific ISO settings is far more important than the limit the manufacturer might have included. IOW, the manufacturer might include a ridiculous high ISO or extended high ISO setting that looks great in their marketing material but images shot at that limit are so noisy that folks just don't use those highest settings. Basically don't use the limiting high ISO setting you can dial in to compare low light performance of two different cameras.
For cameras of similar vintage a full frame camera assuming you can sufficiently fill the frame with your subject (IOW, not relying on deep crops) will always out perform a crop sensor camera or cropping the full frame image in post from a noise and dynamic range perspective once the output images are sized to the same output size for digital display or printing. So if you can fill the frame on a Z8/Z9/Z6 I, II or III or something like a D850, D5, D6, etc. which are full frame cameras you'll have better low light performance than filling the frame on a D500, D7200 or similar crop body camera. From that standpoint alone, comparing the highest ISO setting on the D500 (640,000 boosted) vs the 102,400 on the full frame camera isn't a great comparison as it doesn't really capture the issue.
Yes, if you can sufficiently fill the frame and not have to deeply crop the Z8/Z9 image it will have lower noise and better low light performance than the D500 shot in the same light but of course with wildlife it's often easier to fill the smaller sensor of the crop body D500 so if you end up cropping the Z8/Z9 image to D500 dimensions the advantage disappears once the two images are compared at the same output size. For wildlife work that usually means using longer focal length lenses or learning to get closer when shooting full frame cameras vs crop body cameras to fill the larger FX sensor.
One of the easiest ways to see how different cameras perform in low light conditions is to compare their respective Dynamic Ranges at corresponding ISO settings over at PhotonstoPhotos. These charts normalize for output size (compare final images sized to the same output dimensions for screen display or printing) and the key is to understand that Dynamic Range (DR) in these charts is measuring from the brightest white tones you can capture without blowing out the image to a defined noise floor or acceptable amount of shadow noise. So when the numbers get lower on these curves that means shadow noise is more visible and when it gets too visible (the curves fall to low numbers as in 3 or 4 stops of DR) the image will look quite noisy in the darker regions.
Here's an example of a Z8 in full frame and DX crop mode vs the D500 to show how they compare in terms of DR and from it low light performance:
What this chart shows is the Z8 in full frame shooting has the highest DR and the lowest level of noise in the dark parts of the image for a given high ISO setting. At low ISO settings the Z8 and D500 are almost identical and that jump in the Z8 lines is what comes from a Dual Gain camera design. The D500 comes in a bit lower across the range of high ISO settings and a Z8 in DX crop mode comes in a bit lower yet so a bit more noise in the darker parts of the image or a bit more than a stop more shadow noise than the full frame Z8.
The way the Z8 becomes noisier in crop mode vs full frame mode is why it's important to sufficiently fill the frame and not rely on deep crops if you want to get the most out of the camera's low light performance.
