They are fairly simple. It looks it had a regular waist-level finder so you just pop open the top and look down into it. You are looking through the top lens, the bottom lens is the one that projects onto the film. Aperture and shutter speed usually just set with a knob. Prime the shutter, then take picture, then one of the side knobs usually advances the film. They are pretty simple. Look up twin lens reflex camera guide on google. There were millions of cameras made with this same basic design by every brand imaginable. Some cameras are still made with the same basic design.
If you want to use it I'd advise this: Instead of buying a bunch of color 120 film and then having to send it off to process or go to a local pro lab or something....
Get some black and white film and get some chemistry. Enough chemistry to develop a whole bunch of film should be able to be purchased for under $100. I think here in portland at pro photo supply its like $40 for enough to do quite a bit of black and white film. Then you can just develop it in your bathroom. Black and white development is really simple and despite what anyone tells you, you don't need any equipment other than a stopwatch, a container or some sort (cat litter pan works lol) and a bathtub. A thermometer helps. "But your temperatures won't be right"--it doesn't matter, the bathtub will make a decent enough temp stabilizer.
Then you get to have some fun, play with film, and then when you are done or bored clean up the camera and e-bay it.