Nikon Z 8 Reference Manual page 51

Press the M button to cycle through the displays as follows.
[Automatic display switch]: The camera automatically switches
between the viewfinder and monitor displays based on information
from the eye sensor.
[Viewfinder only]: The monitor remains blank. The viewfinder is used
for shooting, menus, and playback.
The viewfinder display also lights for a few seconds after the camera is turned on and after
the standby timer is activated by pressing the AF‑ON button or pressing the shutter-release
button halfway.
[Monitor only]: The monitor is used for shooting, menus, and playback.
The viewfinder display will remain blank even if you put your eye to the
viewfinder.
51
The Viewfinder