How does the usage of the food and the wheat work?
In terms of wealth? Or in general?
Items sitting in a structure somewhere is what counts towards wealth in 1.2.5. When food is consumed the item is destroyed, and no longer counts toward wealth. Food is used by houses and barracks. Every 600 turns (a little over 2 minutes) a house requests a piece of food, and destroys it when it gets it.
Barracks also consume food, but work a little differently. Each warrior garrisoned in a barracks consumes "supplies", this is just a counter in the building. A barracks will request food, and consume it to produce more "supplies". In general, for each warrior in a barracks, that barracks will need 1 food every 300 turns (75 seconds).
Bakeries consume wheat and produce food, on a 1-1 ratio, taking 10 turns each time. Farms consume nothing, and produce 3 wheat, taking 60 turns to do so.
All of these numbers could change, of course.