NieR: Automata offers extensive free-roaming and side quests between main story missions. Many quests might seem missable on your first playthrough. However, true completion requires revisiting earlier sections after finishing the game. This guide explains how to unlock and utilize the Chapter Select feature.
This article contains minor spoilers regarding achieving the true ending.
Unlocking Chapter Select requires completing one of the game's true endings. This involves finishing all three playthroughs and selecting a specific ending during the final confrontation of the third playthrough. While referred to as playthroughs, many consider them chapters due to their individual narrative focus.
After seeing the credits of a playthrough, save your game. Load that save to begin the next section with a different character. The final playthrough involves multiple character switches; completing it unlocks Chapter Select for that save file.
Access Chapter Select from two locations:
This menu lets you select any chapter to load, retaining your progress (weapons, level, items). You can also choose your character, provided the chapter involves multiple characters.
Note that completed side quests cannot be replayed, regardless of the chapter loaded. Always save at an access point before switching chapters to preserve progress; otherwise, any gains (levels, items) from that chapter will be lost. Chapter Select is ideal for completing all content and exploring different choices to obtain every ending.