This is story related and cannot be missed as the door doesn’t open until you’ve done it. Click the spoiler for details.
*** Spoiler - click to reveal ***You must enter the same sort of machine as the other two flashbacks and bring your human back to the helicopter landing site.
Look for interactable objects and press cn_A. There’s several doors and I believe one lever early on. You can drop off edges safely to progress.
When you come across a locked door proceed over to the left side of the room and up to the next area. Climb the ladder on the left of the activated security system, the key is at the top, go back to the locked door and collect the stun gun inside. Go back and shoot the security system to progress. You can miss and the gun will slowly real in the wires and reload. In the next area you have to unchain two hanging boulders in order to open the door.
Once outside you can jump off the ledge. Run to the furthest helicopter and watch the ending.