Entire books have been written on the question you have just asked. Short answer is raw materials, blueprints, machines and/or hand work, final product.
Like any other product, guns are manufactured to be sold. Buyers can be private citizens, business entities, or governments.
a factory
Raw materials, workers, bueprints.
Every corner of the globe
There is no one single location.
The Iver Johnson Company
HD Folsom or Crescent.
Etronix
Gunsmiths don't make guns - they may assemble them from previously manufactured components, but they don't actually "make" guns. Factories make guns.
Manufactured goodsGuns
they are guns,flags,mailboxes, and coffee
Check the Blue Book of Gun Values