When I encountered these they were maintaned by a community or space like houseprojects, collectives or free shops. Some seemed to be accessible all around the clock and some just when the space was open. So its not just a fridge standing around but actual groups/people caring for it.

I dont know if the places were connected to the food-sharing network, but some probably were. Here is a wiki entry about that https://en.m.wikipedia.org/wiki/Foodsharing.de