Looks like you took the photo using ISO 6400, which explains the noise your seeing.
In general you want to use the lowest ISO the camera offers just like in the film days the lower the ISO/ASA/DIN film the less grain or noise in the photo.
Take a look in your Nikon Z7 II user manual on how to set the ISO.
Any reason for shooting 1/6400s at ISO6400?
The beautiful scene you shot would have been zero noise at ISO100.
Goes to show that you can't always trust auto.
I prefer Full Manual or Aperture/Shutter priority with fixed ISO.