Code Vein - Depths

You can find Maps on your journey that unlock Depths levels in Home Base by talking to Davis (big knight character). He will give you option to enter the level of the map you bring back to him. Each level has keys and boss room which you unlock by using those keys after collecting them. Many levels also have Vestige Parts to find.

You will need to enter all maps to unlock Deep Trailblazer Trophy. I unlocked it as soon as i entered last 12th map, so it seems it's not required to 100% all the maps.

Depths: Den of the Dead - Pick up in Ruined City Underground - Outer Crossroads Mistle. Follow the path nest to Shang through the hallways. It's on the left soon close to a corpse.

Depths: Den of Darkness - Teleport to Ruined City Underground - Outer Crossroads Mistle after defeating Oliver Collins. Talk to Shang who is close to Mistle - Code Vein - NPC Quests Guide

Depths: Town of Sacrifice - Pick up in Dried-up Trenches - Decayed Ship Interior Mistle. From the Mistle, climp up the ladder and find it next to a corpse.

Depths: Swirling Flood - Swirling Flood - Given to you from Shang who is in Ruined City Underground - Outer Crossroads Mistle. It's a part of his Questline - Code Vein - NPC Quests Guide

Depths: Flood of Impurity - During story you will need to interract with Goddess statue close to Ruined City Center - Park Ruins Mistle to progress to your next area. On your way, after climbing the ladder in a cave, look behind you and pick ip up.

Depths: Cliffs of Rust - Given to you by Self-Important Revenant in Dried-up Trenches - Decayed Ship Interior Mistle. It's a part of his Questline - Code Vein - NPC Quests Guide

Depths: Silent White - You need to finish Swirling Flood Depths, do Shangs request there, return back to him, talk to him until he hands over this map - Code Vein - NPC Quests Guide

Depths: Arachnid Grotto - Reward from Gustav NPC Questline. First you can find him next to Dried-up Trenches - Dried-up Trenches Entrance Mistle after certain boss. He will ask you to gather some evidence for him. After doing some and after some more events, he will be nexto to Ashen Cavern - Accumulator Ruins Mistle. Talk to him to get the map - Code Vein - NPC Quests Guide

Depths: Misty Ruins - Pick up in Crypt Spire. This area is pretty straightforward and linear actually. It will be close to first ladder you need to lower for a shortcut down one of the paths.

Depths: Rocky Ruins - Reward from Richard and his Questline close to Ruined City Underground - Natural Cavern Depths Mistle. If you can't find Richard anywhere, you need to finish Questline from Self-Important Revenant first - Code Vein - NPC Quests Guide

Depths: Zero District - Just as you beat Queen's Knight Reborn in Provisional Government Outskirts, follow the path and you can find it close to the ladder.

Depths: Void District - Pick up in Provisional Government Center area.