When a building needs a resource it will search outwards along roads, bout routes and train routes to check each building until it finds the available resources it needs. Once a resource is found the resource will be sent on its way and the destination for that resource is now fixed.
if a resource is coming from a long way away, it will continue on its journey, even if another resource of the same type appears at a closer location in the meantime. This is to ensure you have full control over how resources move about your colony (see Transport Rules ) and your network is not clogged with resources going back and forth based on route-finding decisions that you’re not able to see.
A resource's route to its destination is recalculated every time it reaches a road post, harbour or station. If it can no longer find a route to its destination building it will search for another valid destination building. If no building will accept the resource, it will be destroyed.
At any point you can select a resource to see the route that it's taking and where it's come from.
If you select a building you will see in the main production panel the input resources required. If there is a green circle with a number inside this indicates the amount of resources that are currently en route. These resources will also be highlighted with a green marker in the world.
You can select any resource and the main panel will display where the resource was sourced from and how long ago as well as where the resources is heading.
If you find a building is sourcing resources from further away than you'd like then the best way to control resource sourcing is to use Transport Rules. You can also limit which other buildings a particular building can import to, or export from. More info on the ‣ section.