When you go to the Foundation, you’ll see an elevator ... will see a cutscene of the lift lowering inside the Foundation. You will then be able to ride it to the Basement.

You’ll need to draft it, move the lift down into position by revealing a hidden crank on the back side of the wall, and then take the elevator down where you will find a Basement door.

which is meant to take cars from the basement to the roof. “These lifts were the central feature of the residence to move the vehicles around to the multiple levels where they will be parked ...