When they make steel, they basically take iron and add between .01% to 2.25% carbon. Anything higher than that starts getting into the realm of "cast" or "pig" iron.
when the foundry is making the steel from molten iron, they add the carbon and any other alloying elements... nickel, vanadium, etc. That's when chromium is added. They add the alloying elements to make the particular steel based on what they want the end product to be. All stainless steels have chromium, as well as other alloying elements in it.
The other way (rare) that people get a stainless effect is by taking carbon steel and coating it in chromium... i.e. "chrome plating." This works when serious corrosion resistance is needed but isn't needed inside. This frequently happens with firearms and knives that are made from carbon steel and the end user wants corrosion resistance because the tool will see use aboard a ship and frequently exposed to saltwater. I believe that chrome bumpers and wheels on cars are made in the same manner.