You can start searching for the Sanctuary right after you have found Adam's head and had a conversation with the mysterious figure that claims to be Adam, or just after breaking into the mind of the main character's son and playing out all the memories.
Leave the killer's hideout and head towards the newly unlocked passage that leads to the sewers shown in the image. Crossing the sewers is not too demanding. Shortly after you begin the journey you need to use the lever to open the first large gate.
In the further part of the sewers you need to locate a box with three small switches shown in the attached image. Interaction with each of them will unlock a new pathway (locate the small gate with a green lamp above it). While exploring the sewers, look around for a new collectable photo. You will reach the exit from the canals shortly after moving a second lever and unlocking another large gate.
Take the stairs upwards. They will lead you to a hatch and when you open it, you will find yourself just before the entrance to the huge Sanctuary building. Before you go inside, look around to find another photo.
Upon entering the Sanctuary you will receive a new assignment. Open the middle door and head to the opposite end of the long corridor. Once here, approach one of the monitors and introduce yourself as Daniel Lazarski.
After the identity of the main character is verified, turn around and approach the right-hand screen, which has been shown in the image above. Touch it and enter the newly opened capsule. It will take you to the digital world, where you will have a long conversation with the "virtual" Adam, who will appear on the monitor attached to the largest tree. The game will uniquely allow you to use up all of the dialogue options.