After you have completed the Fifth Sage quest in Zelda Tears of the Kingdom, you’ll unlock the "Guidance of Ages Past" and Spirit Temple where you will hear a mysterious voice asking you to take the mask that you get during the Fifth Sage quest in the direction shown by the green laser. Once at the end, an underground path will open up. Go inside until you reach the Golem Factory. Place the mask in the receptacle and you will hear the voice of Mineru, sage of spirit, sister of the first King Rauru. This will ask you to rebuild the Golem / Robot by finding the 4 missing parts.
Here’s how to find all four parts and rebuild the golem:
Right leg depot
Head to the first entrance northeast of the Golem.
Follow the large dragon skeleton to light up the area.
Head to the northeast entrance to pick up the first missing part.
Activate the mechanism and take the block containing the right leg.
Use the elevator to raise the leg.
Attach 4 fans to the block and activate them to reach the upper level.
Take the next elevator to go even higher.
Collect the chest containing 3 Zonaite and use your glider to bring the right leg back to the Golem.
Left leg depot
Go to the southeast entrance, paying attention to enemies.
Activate the mechanism to release the left leg block.
Use the elevator to go up.
Turn the wheel to lower the ramp.
Attach rockets to the leg block to propel it to the upper floor.
Exit the depot to bring the left leg back to the Golem.
Left arm depot
Head to the southwest entrance.
Activate the mechanism to release the left arm block.
Use the suspended wheel to raise the door.
Attach a rudder and 2 wheels to the block.
Cross the lava using the improvised vehicle.
Climb the ladder to retrieve a chest with 5 rudders.
Change the location of the wheels on the block to cross a new lava obstacle.
Use one of the wheels to raise the door.
Collect the chest with 3 Zonaite in the windmill.
Run down the river with the fans to bring the left arm block back to the Golem.
Right arm depot
Go to the northwest entrance of the Right Arm depot.
Activate the mechanism to release the right arm block.
Use the ramp to go upstairs.
Use motorized wheels to drive the block across the electrified bridge.
Attach motorized wheels and a rudder to a platform.
Put the right arm to the vehicle.
Go down the ramp with the vehicle, being careful of the enemies.
Bring the right arm back to the Golem.
Now that the Golem is rebuilt, it will come to life. Equip yourself with the weapons found on the side to face the many enemies that await you.