https://forum.paradoxplaza.com/forum/developer-diary/development-diary-9-economy-production.1595744/
SPECIALIZED INDUSTRY VEHICLES
Specialized industry vehicles are not part of the normal traffic and do not drive on roads but they still travel on paths created by the simulation when industrial areas are created. The vehicles operate within these specialized industry areas only and gather resources within that area. Once they gather enough, they drop the resources at the main building which then processes the resources and sends them to other companies for further processing as part of the production chain.