The Codex is generated from the game's own files: every building with the figures the in-game Codex shows, the nine resources, the ten kinds of ground, the six night events, the Sundered and all 100 achievements with exactly how each is counted.
The game hides nothing here either: its own Codex states every unlock condition, met or not. Some are only announced in the build palette once you are close, so the Buildings page is the one to read if you would rather find them yourself.
BuildingsAll 54 buildings on their 22 upgrade ladders: cost, work, crews, unlock and what each step buys.
ResourcesThe nine resources and every building that makes them.
The FenThe ten kinds of ground: what can walk it, build on it and work it.
Night EventsThe six night events and what each one really does.
The SunderedThe raiders and the Sundered Places they come from.
AchievementsAll 100 achievements, with how the game counts each one.