First of all, the preparation time is comparable for most dishes in a typical restaurant. Each can be put together in somewhere between 4-10 minutes. Restaurant kitchens do a lot of prep work in advance, all the chopping and sorting things into containers, everything that doesn't need cooking over a flame or arranging on the plate.
Basically, the goal is to convert grocery store ingredients into tasty meals without the convenience of a kitchen full of appliances, utensils, and seasonings. You want to be able to go to the grocery store and get ingredients in small enough portions that you can use them all, without having to buy a million little things.
For the week before the buffet, you have to begin conditioning your tummy. What this means is that for at least one meal a day, eat till you're full (even if it's just bread) and when you think you're full, drink as much water as you can. This is to prepare your stomach to stretch like never before.