To get out of the room with the strongbox and CHALLENGE TOMB ENTRANCE, scramble up the painted metal wall.
Move straight forward to the edge of the ledge, jump to grab the yellow ladder, and climb up to the loft to find an Archivist Map (8/9) and a box of cloth. Climb back down to the floor.
Go up the steps and through the doorway where you burned the first flammable barrier earlier.
Inside, head up the ramp to the next level.
Here you'll find a bunch of very dead Russians, plus some salvage and other supplies.
If you like, you can make a rope line from here to the water tower, slide over there, and collect some salvage, cloth, and feathers. If you do this, you'll need to climb all the way back up to this building again, so you may or may not want to take that detour now.
In the room in the back right (northwest) corner of the same building...
...there's a relic (16/18), a Hero of the Soviet Union Medal from the Red Army Officer set sitting in a fenced-in area on the left.
On the wall above the doorway is another cosmonaut poster (3/7) you can burn using a molotov (or later a fire arrow) for the challenge.
Inside there's a mural (11/12), a Soviet Crest titled For the Motherland which gives XP and Russian proficiency; and various minor supplies.
Before leaving this room, use a rope arrow to pull down the rope-wrapped barrier...
...revealing a long chute.
Slide down and jump at the very end of the chute to land on or grab the ledge on the far side of the barnlike building below. If you miss the ledge, you'll need to climb back up and try again.
Once you reach this ledge, pull up and collect some salvage, plus arrows and more salvage in a locker. Then pry open the strongbox (10/14) to obtain a BOLT ACTION RIFLE COMPONENT (1/4). There's also a cosmonaut poster (4/7) you can destroy for the challenge. There should be a few bottles sitting nearby that you can use to make a molotov.
Climb onto the ledge above the strongbox to find more salvage.
Shoot a rope arrow into the rope-wrapped beam in the building above.
Then climb up there to find a relic (17/18), an Order of the Red Banner Medal from the Red Army Officer set.
Use a rope arrow to pull down the wooden barrier in the celing and climb back up to the room where you slid down the chute. You'll find some arrows and a container of cloth on the way up.