Unlocked by completing the game on any difficulty.

Unlocked by completing the main story on Veteran and earning the Poilu achievement.

Unlocked by achieving S Rank on Veteran Difficulty.
The pantalon rouge (French for 'red trousers') were an integral part of the uniform of most regiments of the French army from 1829 to 1914.

Thanks @Scooter !

How to achieve S rank and how do ranks work:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3300676980

Unlocked by achieving ending 4 (secret ending).

How to achieve ending 4:
Do not abandon your post in the defense of Chapters 2 and 4,then release the pigeon in chapter 6 and finally, finish the final battle.
IMPORTANT: After each chapter, in Andre's memory, you need to go into the cellar and push the cabinet, then check Pierre's diary. You need to write down the patterns drawn at the end of each diary, and the password in the last one. (each play-through is different )Then in chapter 6, open the last cabinet in the weapons storage room (Pierre's locker) and get the pocket watch. Carry it all the way to the end.

Unlocked by purchasing the Trench Raider Pack

Unlocked by purchasing the Trench Raider Pack

Unlocked by purchasing the Trench Raider Pack

Awarded for achieving: Conscientious Objector
Complete the main story without directly killing any enemies with guns, melee, grenades, or hazards.

You need to abandon your post during every battle. In Chapter 1 by figuring out a code (you can find it written on two pieces of paper), in Chapter 2 and Chapter 4 by hiding in small room underneath a trapdoor, and in Chapter 6 by not defending Fort Vaux.

Once you get the wire cutter, you will unlock the Fire Trench and Trench Infirmary locations. As you know, you need to kill a couple of knights and get the Infirmary key from their dead bodies, but you cannot do that. You must collect a 75mm shell and fire it upon the rubble in the Fire Trench.
This way, you get the key without voiding the achievement.

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3301457433

Unlocked by either Pre-Ordering or purchasing the Golden Pack DLC.

Unlocked by achieving S+ Rank on Veteran Difficulty.
How to achieve S+ rank and how do ranks work:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3300676980

You can purchase it for free from the merchant on your second playthrough after completing the Elite Trench Raider challenge, which requires clearing the game using only melee weapons.

As a special weapon, it is a complete weapon with infinite durability and the strongest attack power among melee weapons. With just this, you can knock out flamethrowers in melee combat. (Based on soldier difficulty, you will die after 7 full swings from the front without a rear attack.)