A little bit more on this for anyone else who is as slow as I seem to be (It's the white hair...). First of all, DISP is on Menu/Camera2, page 6
Second, and this is true for the A7r III (don't know about others), on the back of the camera, on the main wheel, the top button says DISP. When I push that button, it toggles through the Display modes that I have checked in the Menu. So, the menu options turn on/off what shows on the rear LCD or viewfinder when pushing the DISP on the back of the camera. I can't show the level and the histogram simultaneously for example, but I can toggle between them by pushing the DISP button.
Fortunately the A7RIII help guide is only 464 pages.
Mike.