Unlike many traditional adventure games, Amnesia: A Machine for Pigs does not feature explicit 'side quests' or optional objectives in the conventional sense. The game is a highly linear, narrative-driven experience, with its primary focus on unfolding the story of Oswald Mandus and the Machine. Every action you take and every area you explore is directly tied to the main plot progression.
However, there are 'activities' that enhance the experience and deepen your understanding of the lore. These primarily involve thorough exploration and the collection of notes and phonograph recordings. While not mandatory for completing the game, engaging with these elements significantly enriches the narrative and provides crucial context to the horrifying events unfolding around you. Think of these as narrative collectibles rather than optional missions.
Compare side quests & activities inARK: Survival Ascended