In Metro Exodus you will find several side missions. For completing them you will receive additional items that can make your gameplay easier.
In Volga you will come across an abandoned building. In the building you will find slaves who you have to save.
To enter the building you have to quietly get to the top, by the stairs.
There you will come across an open passage. Enter it quietly. You can see your opponent sleeping right away. Walk up for him and stun him, and you'll exclude him from the fight.
Go down the stairs quietly. At the bottom there is a dog waiting for you, which you have to get rid of quickly, otherwise it will betray your presence to all his companions...
In order to release the prisoners, you must eliminate all enemies.
After clearing the location, approach the slaves and open their cell. In return for the release, they will give you the key.