The metal of the last stage will have to be collected in a "robot factory". Knowing that, head to Pressure Plant, easily recognizable since a 45-meter giant robot is under construction there.
Once inside the Pressure Plant compound, any item counts to complete the challenge: structures, walls, metal cylinders laid on the ground...
If you have come this far, you have completed the 3 steps of the challenge. It is time to complete the rest of the challenges of week 10 or, if you have already done so, to enjoy a well-deserved rest.