120 & 220 are film sizes, both are 6cm wide, 120 holds 12 6x6cm frames and 220 24 frames
6x4.5 and 6x6 are frame size in cm, there is not different quality.
Backs are the cartridges that hold the film and mount on the back of the camera, their size refers to film that they are design to work with and frame size they produces. If you put 120 film in a 220 back, or visas-versa the spacing between frames will be off, the frames may over lap or the last frame will be cut off.
In the US the cost of film, development and enlargement for 220 vs. 35mm 24 frame are about the same, for both color and B&W
Proof prints color 220 about .50 cents, 35mm between 5 to 25 cents, big cost different
B&W proofs about the same around .50 cents
The two type of shutters used in most cameras today are leaf and focal-plane.
Leaf shutter cameras have a curtain in the camera body that covers the film, the shutter is in the lens, works similar to the aperture.
How it works (very simple put). Your are look thru the camera, the curtain is closed - the shutter open, as you take the picture the curtain opens – shutter closes, next the curtain closes, you advance the film, the shutter opens, start over. The entire film is exposed immediately. You may get motion burr at a shutter speed of 60 with low speed movement
Focal-plane shutter cameras the shutter and curtain are the same and are located in the camera body. Them you fire the camera the shutter/curtain open a slit in itself, the slit is padded across the film. The film is only exposed when the slip is over it. Motion burr is less common. Low speed movement will not burr 99% of the time. High speed will and can be made to if you want to.
The only book I have on medium format cameras is for Hasselblads. “The Hasselbled Manual” 4th edition by Ernst Wildi
If your store has a copy of it the shutter info is on page 5 of it.