The Z6 is the "high ISO" champion in the Z range -- so far.
A quick demonstration of how it handles 50K ISO (51200), 80K ISO(eq.), 100K ISO(eq.) and finally 200K ISO(eq.). The 50K ISO (51200) rating is the upper end of the factory calibrated ISO settings, the remainder correspond to Hi0.7, Hi1, and H2, respectively.
All with the 50/1.8S on the Z6 a dark, cold winter night. Hand-held, f/9, and shutter speeds starting around 1/60 sec. NEFs run through Photo Ninja at flat settings expect for the noise reduction, that was set to 'moderate'. Downscaled and converted to sRGB before posting here.
My impression is that one can, given conducive situations, get a lot out of the camera even at these elevated ISO equivalents.