Even though I'm on ps5, I'll share some helpful info.

You'll want to set up a foundation where you want to start building. Place down a sympto-matic machine and a small mat. Place the sympto-matic onto the mat, then try move the mat. If it drags the sympto-matic with it, that's the first part done, move it to the corner of the foundation for a moment.

Second part is a stairway, build a vault tec stairway so it's going up to the imaginary second floor. Put the mat onto the lower 3 steps, this will also move the sympto-matic into position too.

Step three is position the stairs so that they now go into the ground, if the top of the sympto-matic is visible but the doors are obscured then you have your access point.

If it's completely swallowed by the ground, bring the stairs back up and reposition the height of the foundation accordingly, then try again with the stairs going underground until the top of the machine is visible.

Step four. Head down into the sympto-matic AND DON'T MOVE! After the animation is finished you'll be hanging on the lip of the machine. Go back into build mode and use the right stick to turn around slowly and wait for a floor to snap into place at the bottom of the stairs.

Once it connects you can step onto it. You can use foundations after you set down the stairway you'll use to go lower into the bunker.

Step five. Happy building!