Throughout the movies, we see that there are tons of models that are manufactured by Skynet and so, one would have to wonder where all this metal material is obtained from.
The backstory may be that Skynet was an assembly line factory, like a car factory, where all it made were robots; the U.S.'s newest attempt at making robots that would fight for them as a nation to limit the amount of human casualties. Of course, since it went rogue, all the material left over is being used to manufacture units that are reprogrammed by this glitch or virus that is seen to give advanced AI to Skynet to program these machines that want to kill all humans.
So where does it obtain material after it seemingly should logically run out of metal to make its machines? Heck, how does it even start making liquid mimetic poly alloy machines with leftover materials?
I guess to start the discussion, one could insist that there are many units that go out and recycle blown off, but still usable Terminator parts from previous models.