Trucks in Warzone are something of a mystery; it is never explained how they work. However one may guess based on technology that is currently being explored how they might operate.
One possibility is 3d printing which operates by laying down a thin layer of material, then sintering it with a laser.
Another method would be to use externally powered and controlled nano machines; externally powered and controlled because it is unlikely that independent nano constructors would be widely available before the collapse. This theory is supported by the fact that nano manufacturing is one of the techs you can research. It is not unlikely that this technique is used in addition to 3d printing, though.