This is a belt, a big loop basically, where requested items circulate around until taken. The green wires on top are there to count items on the belt; this isn't totally necessary but helps avoid loading too many items. The red wire connected to the poles is the requester circuit; circuit connections are really simple.
Build your supply factories. Connect an inserter from each type of resource to the red wire, and enable the inserter when its resource is > 0.
Here's an example of a single demand. The green wire coming from the north carries the request: 100 mining drills. The filter-inserter is connected to this wire, and configured to "set filters" from the wire. The combinator is simply set to "each + 0, output each"; I call this an isolator. It's there so that the local demand (100 drills) can be added to the red "total demand" wire (which combines all requests).
Lets go back to the boxes: There are only three types of combinator used:
- Constant combinators, to set the desired contents of each chest
- Arithmetic combinators, set to "each * -1; output each"; I call this a subtractor
- Arithmetic combinators, set to "each + 0, output each"; I call this an isolator
- green wire from box to input of subtractor [carries box contents]
- red wire from output of subtractor to constant combinator, filter inserter, and input of isolator [carries constant combinator values - box contents]
- red wire from output of isolator connected to red requestor wire
Here's a blueprint for the boxes. (They should be passive provider boxes, but I haven't built any of them yet in this game.)