How does Nikon d7000 focus? What effect does it have on the camera screen? I only have a square.

39 focus points are displayed simultaneously in the viewfinder?

There is only one possibility: in the mode of autofocus plus autofocus area, when the camera is perpendicular to a very flat plane (when the focal plane is parallel to this flat shooting plane), you may see 39 focus points (not necessarily 39) at the moment of focusing. But is it necessary?

39 points of focus does not mean that 39 points should be used every time, but that there is a lot of room for you to choose, 39 points.

If the autofocus mode is selected, the camera can provide 39 focus points to choose from. No matter what kind of focus area, some machines automatically choose and some let you choose (single-point area focus).

In the focus screen, only a few or one point that plays a role in each shooting can be seen flashing.

Page 95 of the instruction manual mentions that no matter which AF area mode is selected, "only the current focus is displayed in the viewfinder".

So your camera shows normal.

I don't know the source of this picture, but it may be a sample picture processed in an evaluation, and it is suspected that it is not a real shooting scene.