They setup a situation.. in this case, they contend there was a biological war. They strand 8 people in the middle of an area. In this case, it's about a 12 acre area. so big enough that they can't walk out in a day. The people are then responsible for coming up with their own food, water, electricity, as well as defense.
They are free to raid the buildings or areas around them and determine where to eat and sleep. Since it's a city area, they have to figure out their own sanitation and water situations.
A lot of it is invention.. like "how do we get electricity" "how do we get food". The show sends in "raiders" every so often who will torment the people. The raiders are just scripted people... with the authority to beat the s--- out of the people involved, destroy their goods or steal from them. So, late at night, a raider may come in and burn something, steal something, or just wake them up to prevent them from getting sleep.
For the duration of filming, they do not have "challenges" or real contact, they decide the direction the show takes. Only if something medical emergency comes up is there intervention.
It's.. different. This last week, they were desperate to get power. The week before they found a truck full of rotting pigs (pigs covered in maggots, etc. that was a few weeks old and rotten). While they rotated puking and dragging, they managed to get the meat so they could cut the fat off of it and boil it into eco-diesel. Pretty innovative.