You can chose to enforce strictly even input/output product ratios. If you enable strict output ratio, products will be output to all output ports evenly. If a port cannot accept any more products, output will pause until the port gets freed-up. Input strict ratio works in a similar way.
Assigned trucks will follow this harvester which will load them with wood. Trucks will deliver the wood to the factory and return back to the harvester.
Assigned vehicles will be allocated to transfer cargo exclusively for this building. This means that anytime an assigned vehicle performs a delivery this building has to be either source or destination of the cargo. This can be combined with routes assignment to form dedicated supply lines.