Commercially available camera systems, include multiple cameras and a central connection point which I can only assume is an electronic switch to allow the viewer to pick which camera to view.
I see these muli-camera systems on sale all the time at various electronics houses. That's probably what you should have purchased.
You have the cameras, but you don't have the electronic switch, to select which camera to output to your Monitor.
As I've said many times before, Google is your friend. If you just Google the topic you'll get over a million hits. Here is just one that I picked at random.