A bunch of clever stuff goes into that! The entire world isn't really there at one time. Only the area you're in, and it will dynamically load the other areas when you get near. Reuse of assets and textures and stuff can make things way more efficient to store (e.g. storing one grass texture for every grass block in minecraft). Storage is just really optimized so you don't have to save a lot of data to represent the state of the world; just enough to rebuild world when you need it

I loved it when I was a teenager. I despised Holden Caulfield, but also related to him on some level, and that pushed me to really think about my own position in life and thought processes. Now, as an adult, I still love it. I feel like it beautifully stresses the importance of having good emotional support structures and mental health awareness for when your life goes through a rough patch.