Common Design in a Nutshell
Common design is observed all throughout our technological world.
That is because of standards that allow technology to work together.
For example many different printers will work with one PC- that is due to a common design.
Most PCs are very similar due to a common design.
Most cars are very similar due to a common design.
The list of such things is very long.
There isn't any need to keep re-inventing things, most times there are standards that must be adhered to and those standards drive the common design.
Designers can (and do) also take a part being used for one function in one system and use it for another function in another system.
So that is common design in a nutshell...