Half-Life: Alyx | Chapter 4: Superweapon
You need to teleport to the ventilation shaft where you just got the resin, then teleport along the pipeline until you reach the passageway across the room, next to a slightly open door on your right. Open the door, and you'll see a locked door that requires a yellow keycard.
Find some shotgun shells on a cabinet near the steps, on your way to the security door with the yellow lock. You don't need the yellow keycard after you unlock the door, so you can discard it. Continue inside and you'll find a headcrab zombie and an armored zombie. Kill them both and call the elevator.
Go up to the second floor and through the hallway. Don't worry about the crab in the ventilation shaft above you for now; you won't have to deal with it until you find the blue keycard. Turn left at the end of the hallway into the office.
Turn right through the door into the next room and look left. There's some Resin in the trash can on the floor, and the desk drawer with the ammo magazine sticking out also contains the key to the next area—a blue keycard. Check the cabinet to your right for a health syringe and some pistol ammo in the nearby drawer.
On your way back to the elevator, you'll encounter several headcrabs (including an armored crab) crashing into the ceiling in the hallway. Kill them and loot the pistol clip they drop. Return to the door with the blue card in the security office and use the blue card to open it.
Just behind the door, you'll find a locked container (containing four grenades) and several grenades lying around. To use a grenade, pick it up and press the bottom (the "A" button) to arm it. As with other physics objects in Half-Life: Alyx, you can throw grenades in any way you like, depending on what movement you physically perform with your VR controllers when you release the grapple button.
Proceed to the next room and you'll find plenty of grenades. You can carry up to two at a time. The container holds an unlimited supply of grenades, so don't worry about hoarding them right now. Clear the boarded-up doorway, then exit into the passageway with two full boxes of grenades. Throw as many grenades as you need to kill the zombies and save ammo. There's also some resin on one of the pipes below the passageway.
Head to the other side of the path where you'll find a supply cabinet and a ladder. The supply cabinet contains ammo, so grab it before heading down the ladder. At the bottom of the ladder, you'll find a group of Suckers hanging in front of the barrel of an open vent with Resin inside. Attempting to grab the Resin before the Suckers are dead will result in one of them eating the Resin, which you can obtain by killing the Suckers.
Go through the door on the left and you'll find another ammo crate, containing a pistol magazine and some resin hidden behind a sheet of plywood. Turn around and head back to the stairs; you'll find a dead zombie sitting in front of the resin under the stairwell.
The door will take you back to the room where you threw grenades at the zombie crabs, so hopefully you've killed them all by now. Another headcrab will jump out of the open vent above you, opening a passage to the next room, and another zombie will be trapped inside.
Turn around and go left past the locked container. You'll now see a dim corridor ahead of you that turns left, with a luggage carousel. You'll find two Resins inside. Now it's time to return to the locked container and open it. This is the container that drops the grenades. What you should do next is try to throw a grenade into the open hole from which the headcrab jumped out at you a few moments ago.
If you place a grenade in the vent, it will fall into the next room and blow up the locked door, also killing the zombies inside. Now you can proceed through the door next to the large blue shield, leading to a room with a security camera and a weapon upgrade station. Additionally, there are several shotguns on the floor to the left of the security camera terminal and a Resin hidden in a nearby document storage drawer.
Now it's time to face your first Combine Heavy. This is a stronger version of the Combine soldier you encountered earlier, protected by a personal force field that makes it difficult to attack from the front. They will also use a blinding attack if you stand directly in front of them. Try to kill them with a few headshots from the side, as they turn quite slowly. Whatever you do, avoid getting within range of their weapons. Note that there's a Resin in a crate directly in front of you if you take cover behind the column.
Once you've killed the shield-wielding soldier, go around the right side and grab the grub from the locker. Then return to the hallway and use the multi-tool on the sensor in the electrical cabinet. Reconnecting the cable will open a nearby door containing a medical station, a locked container of resin, and ammo. Your actions will trigger an alarm, causing another Heavy Soldier to break through the locked door in the hallway. Kill him, then proceed to the train platform.
Fight your way down the train platform, noticing a medical syringe and a grenade launcher in the cabin ahead of you. Continue down the platform and turn left to find another medical station and a ladder. Climb the ladder and you'll find yourself face to face with another group of Combine soldiers. There are syringes and grenades at the stand. After climbing the ladder, you'll find several shotguns in the row of seats to your right.
After walking down the corridor, continue up the stairs and you'll find yourself in a large tunnel. The exit is nearby, but before you leave, go all the way to the end of the tunnel (where the Combine soldier's corpse lies) and pick up two more Resins.
Teleport through the open window of the nearby hotel, and you'll find yourself on a landing leading inside. Next stop, the North Star Hotel.