To unlock the Every Nook and Cranny in Titanfall 2, you will need to find all 46 Collectible pilot helmets in the game's campaign. If you miss any in your first playthrough, you can collect them by using mission select.
You can find a video below showing all of the locations (credit to Indecisive Prophecy on YouTube):
Timestamps for each level are below:
*** Spoiler - click to reveal ***The Pilot's Gauntlet: 0:13
BT-7274: 0:45
Blood and Rust: 1:35
Into the Abyss: 4:42
Effect and Cause: 9:06
The Beacon: 11:38
Trial by Fire: 15:52
The Ark: 17:04
The Fold Weapon: 18:00