What is the cheap (copper color) metal underneath items plated to look like silver?
It is indeed copper, but a cheap alloy instead of the chemically pure copper that's needed for high quality cables and circuit boards.

I cannot say exactly of which different metals it consists, but it usually includes lots of copper (as the base), nickel, pewter, sometimes zink or lead and sometimes traces of other elements like phosphor, beryllium or silicon.

The copper used for such alloys is usually low quality scrap metal from old copper water pipes or copper that was contaminated with other elements. Electric wiring for example contains mostly pure copper, but if you burn the insulation off instead of removing it mechanically, you contaminate the copper and it loses much of its value. The purification is too expensive, so it's used for cheap metal jewelry and other objects.

Relative to other metals copper is still expensive, but the combination of price and material properties makes this copper alloy the best option for cheap metal jewelry. 


It doesn't rust (at least not as fast as cheap iron based alloys)
It's not toxic (although quite a lot of people are allergic to the nickle in it)
It's not so soft that the finished product bends out of shape at the slightest provocation
It's not so hard that you need lots of heat and energy to shape it during the production
Most people prefer jewelry in a silvery color. Alternatives like actual silver or white gold are much more expensive. Other metals like steel or platinum have a muted, grey color and are harder to process. Aluminum, which has the right color, is much too soft and requires very much energy to smelter and process.