Don’t know if this is a stupid question but where did they get the metal?
Processed it themselves, or traded for it. The ruins of Tiahuanaco show that the “staples” that held the top row of stone walls together were made by pouring molten metal into the slots carved into the stones for that purpose.
There is quite a bit of ore in the Americas and a good deal of the copper, silver and gold were all mined by the American Indians. They did not mine iron and they did not usually mine tin, possibly because it is a rather rare metal. However they mined some tin in this area because a few pieces of bronze have been found.