This chapter of the guide to Metro Exodus has a walkthrough for Sam's Story DLC. To find out if his loved ones are still alive, Sam leaves Aurora and goes east to Vladivostok.
After the cut-scenes, pass the car wreckages and go forward. At some point, you will hear screams - enter the building and you will encounter two bandits. You can kill them - this will not affect the ending.
Collect ammo from their bodies, then run up and climb the ladder to the roof. There you will meet the Captain, and you will find a very important choice that determines the DLC's ending. If you care about a good ending, lower your weapon when the Captain asks you to do so.
Then follow the Captain, take binoculars and watch the submarine. Slide down the rope and you will get separated from the man. Keep shooting the demon - this triggers a cut-scene during which the monster will push you down. Follow the path and enter the ruins.
Go through a narrow passage - the locals will appear. Time for a longer fighting sequence. Press the buttons at the right moment to get out of the fight alive.