Doom will have players finding a number of different colored keys as they make their way through the game’s levels. Here’s how to get the yellow access key in the second level.
As you approach the door that requires the yellow access key to pass, you should spot a small panel to the left of it. Press R3 to activate a hologram of a guy wandering over to the right of the door. Head down this pathway and you’ll find that the path comes back on itself sharp. You’ll need to head down here, but be careful and get your timing right to maneuver your way through the moving parts of the machinery. You should see a little gap in-between some fencing, head through there and take a left. Again, you’ll need to wait for the machinery to move to reveal a small entrance for you to head through. Once you’re in here, take the elevator up.
As you head out of the elevator, you’ll hear the screams of the demons from below you. You’ll want to go left here and then drop down onto the platform just below you. From here, you should be able to see a Gore Nest, make sure you’re fully healed and well equipped. When you’re all set, destroy the Gore Nest. A couple waves of demons will come out of nowhere so be keep moving and unloading rounds from your shotgun into them. If you spot a large group of them, throw a grenade into the middle of them while you have the chance.
Once these guys are all cleared out, go towards the wall nearby where you dropped into this area. You should find another door which opens up to take you on another elevator ride upward. As you step out of the elevator, the hologram from before will show you the way. Follow it briefly, but make sure to grab all of the health and ammo lying around, as well as the Field Drone to get yourself another weapon mod. Once you’ve grabbed everything, go the only other way you can go and spot the body propped up against the wall. Head over to it and hit R3. Your character will grab the yellow access key, but be prepared; as you go to grab it, the body will come to life. A charged shot from your shotgun with either mod equipped should take care of it though, so it’s not too much to worry about.
With the weird demon creature taken care of, head back down the elevator and come back the way that you came, unlocking a couple of security doors on your way back to the yellow access door. Head over to it and you’ll be able to walk straight on through and continue Doom’s second level.