Thanks for all the info. I'm a little slow and understanding it still though, so let's take the following example:
City A has 1 farm.
City B has no farms.
I create a Caravan from City A.
I just send it to City B and leave it there.
For the rest of the game assuming nothing changes, does City A still produce 1 farm worth of food for itself, in addition to 1 food for city b?