I don’t know how they do that, I have seen videos of people actually ringing up a negative balance and being given some more free food because the store is not allowed to pay out cash for someone to take food out of the store! I don’t think it works in this state, at least I have never seen any way to do it here. At any rate the only way to do it would be to just shop strictly for what is available at no cost and then store it till you need it or trade it to someone else for something or give it away to a food pantry. There is no way to take a list of what you actually need for the week and do that.
I wouldn't have the time or patience to do something like they do but these people, like I said, are almost obsessed with this lifestyle. My sister, for instance, brags that she hasn't paid any money out of pocket for hand soap or paper towels in over ten years. Her entire garage is full of shelving, holding all of the stuff she buys and stores. Why you'd want a ten year supply of some of the stuff she gets, I don't know. She used to share some her bounty with my brother and I when I lived near her.