I will explain below with the spoiler tag how to get the cube, if you want to know the general location its between Chamber 21 and 22.
what is the cube for? no idea, maybe just a secret / easter egg.
you can’t take it with you to chamber 22 because theres a grill before the entrance.
After beating chamber 21 walk along the railway and deal with the surprise turrets on the right.
Portal yourself inside the room where the turrets were and look up, you’ll see the cube in an unreachable place.
place a portal on the ground next to the wall that leads to the cube and go to the other side of the room, you’ll see the wall of the other room that you use to the proceed to chamber 22.
Crouch so you can place your other portal at the highest point at that wall and jump through the portal on the ground, while falling shoot the portal with the same color you shot at the wall and that makes you go out the ground portal with enough momentum to get the cube.
picture i took after i tried to bring over to Chamber 22
it doesnt make any sound like Portal 2 companion cubes.
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