Question on modular modeling

Hi all struggeling to find the words to describe what i want to know.

Basically im doing a model in blender for a posable minature figure which i intend to get 3d printed as a prototype for a board game i am working on. im still very new to blender, and art in general, alas i need to learn these things since i dont have the money to pay someone lol.
so far in blender i have drawn a basic head shape with a connection peaice for the body to attach to i want to take the botom of the neck and invert and mirror it to start work on a seperate torso peice. how would i go about doing this?

hopefully these pictures help explain what i mean.

Thanks in advance, Brett

In edit mode select the common faces, duplicate them (Shift+D), then separate into a new object §.