This page of our Doom Eternal guide lists all secrets/collectibles that are hidden in Level 13.
Remember, if you missed something, you can return to this mission at any time. However, you have to finish the level in order to save your progress.
This chapter describes only locations and ways of getting the collectibles. Mission walkthroughs are in a separate chapter of this guide!
You've reached the final codex entry in the game. All you have to do is go to the top floor of the building.
To reach the top, climb up the ruined walls and break the glass to open the path forward.
The room contains the final codex entry - add it to your collection.
To get another heart you have to go up using climbing walls found in the elevator shaft.
Upstairs, look up to the second elevator shaft - there's a ventilation shaft there. Jump in and proceed all the way to the end. That's where you'll find your extra life.