TORNIO, Finland — You must have a very good reason to toil in iron-melting heat an hour’s drive south of the Arctic Circle.

But there is an economic logic behind the Finnish steel maker Outokumpu’s huge complex in this small city, where reindeer fillets are regular fare. Its centralized production line of high-grade stainless helps to explain how global supply chains work — and why President Trump’s trade war could be disruptive.

Outokumpu’s story is one of finding comparative advantage and efficiencies. And it begins hundreds of meters beneath a Lapland forest inside a man-made warren of industry. On a recent day, a miner, Kalle Kilpelanaho, was perched in the cab of a large boring machine deep inside a tunnel, twisting a joystick that could maneuver a drill 20 meters into the rock floor.