Here is the full list of all 16 The Answer Is 42 achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
Welcome to the journey
Complete The Guide chapter
Complete Time is an illusion chapter
Complete How many roads chapter
Complete Deep thoughts chapter
Complete Ultimate question chapter
Find the Ursa Major Constellation
Find the Scorpio Constellation
Find the big answer
Find hitchhiker's thumb
Find a big star
Find the rocket
Find the windmill
Find the fish, thanks for it
Find the Portal - to a new adventure??
Finish all levels in the game