NieR: Automata allows extensive exploration and side-quest completion between main story missions. Much of the game's content might seem missable on a first playthrough. Even after seeing the credits for the first time, the game is far from complete. Accessing all side quests requires finishing the game entirely. Here's how to unlock and utilize Chapter Select to achieve this.
This section contains minor spoilers regarding 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 each a distinct chapter in the overarching narrative.
After viewing the credits of a playthrough, save your game and reload that save file to begin the next section, playing as a different character. The final playthrough involves multiple character shifts, and its completion unlocks Chapter Select for that save file.
Access Chapter Select from two locations:
This menu allows selecting any chapter to load, retaining all progress, including weapons, level, and items. When loading a chapter with multiple playable characters, you can choose which character to play as.
Note that completed side quests cannot be replayed, regardless of the chosen chapter. Always save at an access point before transitioning between chapters to preserve progress; otherwise, any gains in levels and items will be lost. Chapter Select is invaluable for completing all content and revisiting choices to obtain every possible ending.