Short answers, pulled from the story.
The fabri were workers, craftsmen, and artisans who served within Roman legions as specialized engineering units. Early army structure descriptions attributed to king Servius Tullius state that two centuriae of fabri served under an officer called the praefectus fabrum.
Engineers completed the bridge over the Rhine River in only ten days during the Gallic Wars. The structure stood more than 100 meters long across the water despite challenging river conditions below.
Immunes were a class of troops excused from regular combat duties to focus on technical tasks for engineering forces. They led groups of engineers known as architecti and requisitioned manual labor from soldiers at large when needed for heavy lifting or digging.
Michael Grant credits the Roman building of the Via Appia with winning them the Second Samnite War through improved movement capabilities. These routes allowed swift reinforcement and resupply while providing easy retreat if necessary during combat operations.
The massive double-wall stretched over 16 kilometers around the settlement to prevent escape by Celtic leader Vercingetorix and block attacks by reinforcements. One inward-facing section prevented offensive sallies while an outward-facing portion blocked external threats.