Obtaining intelligence from the resistance headquarters is one of the main objectives of mission 1 (The Atlantic Wall) in Sniper Elite 5. The walkthrough explains, among others, where you can find the hideout and how to secure it, where is the safe, and how to get the key to the safe after reaching the building's attic.
Find the Resistance safe house
The intel acquired from the Resistance headquarters is one of the two main objectives of mission 1. You can do this quest after completing the introduction described on a separate page - Securing the beach and helping the resistance movement.
The game will not mark the resistance hideout immediately - you will automatically discover it as you travel around the map. You have to reach the main town and the building you are looking for is located in the south-west corner of the map.
You can approach the hideout, the yellow building, from two directions - from the east or from the north-east. Both of these variants involve traveling through the main streets of the town, so you should make sure that you deal with at least some of the nearby enemies in advance. It's also a good idea to complete the side objective Kill List Target - Steffen Beckendorf - this character is patrolling the city in a vehicle by default.
The resistance headquarters is occupied by enemies. You can try to eliminate some of them with long distance shots, or you can deal with them one by one after getting closer to the building. In the latter case, scan the area with focus and distract enemies, e.g. by throwing bottles.
Recover the Resistance intel
Enter the building and use the stairs leading to the second floor.
In one of the bedrooms, you will find a safe hidden in the floor next to the bed (picture above). You will need a key to unlock it.
Leave the building and check out the front wall. In the place shown in the picture 1, you can start climbing the vines. You will reach the shutter leading to the attic, where you should examine Marcel's corpse from picture 2. You will get Marcel's Key.
You can return to the lower floor through the hole in the floor presented in the picture above. You can pry the boards slowly or use the crowbar to destroy them faster (a more noisy method).
Go back to the safe, use the obtained key on it and take the intel from the safe.