tunnel districts are massive structures found underground under plains, meadows, forests , and jungles. and generate with decent rarity, at an average separation of 13 chunks at a Y level of -32 to 32.
small room with a single skeleton spawner
small room with a single zombie spawner
the ancient loot cache is located in the exact center of the structure, and contains loot from the "shitty_enchants:chests/halls" loot table which will always include a tunnel district log
fake walls are parts of tunnel districts that appear to be endpoints of the dungeon, but instead hold a room with a chest and some glow berry vines, behind them which contains loot from the "minecraft:chests/simple_dungeon" loot pool