Where is the taxiway between the Boeing factory and Paine Field airport?
That "road" is in fact a taxilane, and marked as such on the Paine Field side (note the yellow lead lines marked at the bottom of the image below, you would find on a taxiway). It connects the Boeing plant to Paine Field's "Boeing Ramp" where final prep can be completed before the aircraft are flown off to wherever they're going.



The terminology is important here: The bridge acts as a taxilane (a non-movement area not under the control of ATC which connects various aircraft parking areas to the taxiways). Boeing coordinates their own movements between the main plant (north) and Paine Field's Boeing Ramp (south). (The bridge is also not depicted on the airport diagram and may not even have "taxilane" status - as far as ATC is concerned. It's just sort of "there".)