Following noise evaluation for an outdoor scene, here are two pages evaluating noise with the Nikon Z7, witih one of them compared to the Nikon D850.
Nikon Z7: Astrophotography at ISO 180 Without Long Exposure Noise Reduction
Nikon Z7: Astrophotography vs Nikon D850 with/without Long Exposure Noise Reduction
Includes actual pixels crops and up to full resolution images.
Starry Sky near Saddlebag Lake
f2.8 @ 30.0 sec, ISO 180; 2018-10-04 22:47:34[low-res image for bot]
[location “Saddlebag Lake”, altitude 9950 ft / 3033 m, 34°F / 1°C, LENR=off]
NIKON Z7 + Zeiss Otus 28mm f/1.4 APO-Distagon