In short, by the community. The community fridge near me is refilled mostly by:

People who have to buy more than they want (ie pack of four items, but only need two).

People who made to much food/leftovers, or who just want to cook for others

Weekly community cooking event.

Overproduction from community gardens.

Spillage from restaurants.

Spillage from the grocery store.

People who want to cool their stuff