This walkthrough for Half-Life: Alyx – Chapter 9: Revelations will help you and show how to unlock the door 6 at the end of Chapter 9.
Chapter 9: Revelations Walkthrough
Sewer and Gate
Get out of the sewer and kill the soldiers.
You will see the ship standing in the air.
You have to open the door, but it’s locked. To unlock, go to the observation area where you killed the soldier and press the button:

Kill a quite high number of creatures You need to be careful! You also have to kill the soldiers.
Ruined Building
Go up the stairs in the ruined building. Then you will see a long ladder. Climb to the top. There is a cramped and ruined area here. Look at the gap, someone is talking. When the conversation ends, crouch and be stealth immediately so that the soldier will pass around. If you cannot protect your stealth, the soldiers will fire on you. Meanwhile, the wall in your location will be broken because of fires.
After killing the soldiers, loot (health) on the floor and go down the following gate:

There is a passage covered with boards on the floor. Remove the boards and switch here:

Kill the zombies.
Building
Keep walking on the field and find the elevator arm. Also here are the creatures and watch out:

Get up by the elevator. Walk on the field and you will see the soldiers. Kill them and open the hologram. You will get magazines:

Also you will find this and turn around it. Required to cross:

Access the weapon upgrade on the lower floor of the building:

Make the reflected shapes as follows:

Upgrade your weapon depending on the number of resin you have.
Train Rails
Creatures will appear in the area where the rails are. To be able to defeat them easily, fire the barrels otherwise your bullets are not enough.
How to Unlock the Door: 6?
You will reach the door with 6 on the top of the rails. Use the button in the corner to unlock the door:

It is not so easy to unlock the Door 6. At first, I turned to the lower lock, it did not. Then I turned to the middle lock, it did not again. When I turned to the top lock, it is done. But I couldn’t solve the unlocking method. Finally, the door opened, but it was a coincidence. Do the method as above and also do the following:
- Bottom: Complete
- Middle: Complete
- Top1: Complete
- Top2: When you’re done, turn down the lock path in the middle of the superior.
Hope these screenshots can help for the top:
Top 1:

Top 2:

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