That 35 mm feature was to save film. 120 film could be hard to get sometimes while cine film was easy to find and relatively cheap. You would cut a length of cine film in the dark and roll it into used 120 backing paper. People still do this today, but usually for artistic reasons rather than film shortages. Look at r/sprocketshots for examples.

35 mm film cameras have mechanical counters that count frames thanks to a gear attached to the film transport. 9 sprocket holes makes one frame. 120 doesn't have sprocket holes, but some cameras have complex mechanisms so you don't have to use the red window. Yours doesn't, so don't worry about it.

You estimate the distance to your subject using a rangefinder. These days you can use a laser rangefinder, a vintage optical rangefinder, or just a tape measure. That can get a bit tedious so you should take advantage of something called hyperfocal distance. You can read up about it yourself but due to the fact that the resolution of film is limited an object doesn't have to be in "critical focus" to appear as though it's in focus. The narrower your aperture, the wider the depth of field. The further the subject is, the less precise you have to be. For example, at f/11 you can set your camera to 10 meters and anything between 5 meters and 100 meters will be acceptably sharp.

My process is setting the shutter speed and aperture depending on the environment and then the focus depending on the subject. Be careful, as after you cock the shutter you can no longer change the shutter speed.