Doom Eternal features the following Collectibles: Cheat Codes, Secret Encounters, Modbots, Sentinel Batteries, Sentinel Crystals, Toys, Codexes, Albums, Praetor Suit Upgrade Tokens, Runes, Slayer Keys, Slayer Gates, Empyrean Keys, Mastery Tokens.

They are needed for the following Trophies & Achievements:

- Treasure Hunter
- Master of Fasting
- Running Up the High Score
- Crystal Craving trophy
- King of the Crystals
- They're ALL My Favorite
- Homemaykr trophy
- Extra Extra Lives
- Bonus Stage
- Breaker of Gates
- Gunpletionist
- Meet Your Unmaykr

Nothing is missable. You can get everything through Mission Select. Once you reach the end of a level you also unlock fast travel for that map, allowing you to go back immediately and get missing items. The Praetor Suit Upgrade "Item Finder" is very useful as it reveals all Collectibles on the map. Make sure you play Standard Mode and not Extra Life Mode (Extra Life has no Mission Select, so no way to get missing Collectibles in that mode).

Also do the Mission Challenges (from 3rd Mission onwards) as they give you Batteries that you can use in the Hub Area 'Fortress of Doom'. This unlocks additional Collectibles (you can revisit the hub area after the story via the pause menu).