Fleet vehicles work mechanically according to their individual designs, just as other cars, vans, and trucks do. What makes them a fleet is the system around them: an organization selects vehicles for its work, assigns their use, keeps them supported, and uses records to make future decisions. There is no special fleet engine or body shared by every vehicle.
Consider a business with two equipped service vans and a shared car. The vans carry technicians and tools; the car serves lighter trips. A common record of capability, bookings, and condition allows the business to choose suitable transport, take a vehicle out for repairs, and plan cover. That connection between the physical vehicles and the management decisions is how the fleet operates as a system.