How are vias manufactured, and why tungsten (I suppose it has something to do with high fusion point?).

They etch holes in the oxide for the vias and then use CVD (chemical vapor deposition) to put a tungsten film over the surface, filling the holes. Then they polish off the excess tungsten with CMP (chemical mechanical planarization). I think there's also an adhesion layer of titanium. I think tungsten vias replaced aluminum vias because tungsten with CVD filled vias better than sputtered aluminum. But then copper vias were used because it has better conductivity and better electromigration characteristics.