Zikui, Shared Ownership, and the Communal Food
The food used for an eruv is acquired for the participating residents through zikui. The person making the acquisition and the person who lifts it must meet the halachic requirements.
Acquiring the food for everyone
The owner gives the food to the participating residents through another person who lifts it on their behalf. That act of zikui must take effect before Shabbos and must be performed by someone who can acquire for the others under halacha.
The food becomes the shared property of the community for the purpose of the eruv. It is not enough to designate food that remains solely in one person's ownership.
The food must remain suitable and available
The amount and kind of food must meet the halachic requirements for the number of people included. It is kept in an accessible place within the eruv and must still exist while the community relies on it.
If it is consumed, spoiled, removed, or reduced below the required amount, the communal joining may need to be renewed even though the poles and wires remain unchanged.
A long-term communal arrangement
A city eruv is usually established for an extended period. The acquisition is therefore made with future residents and ordinary changes in the community in mind.
The supervising Rav must know what the original zikui covered, where the food is kept, and when a new acquisition is needed. This quiet legal element is as real as the visible boundary.