A method for operating a manufacturing system (1, 10, 15, 18, 21, 22, 29) for joining, transporting and handling preassembled structures (7, 7'), in particular motor vehicle body subassemblies, which are formed by components (6a, 6b) which are mounted loosely beforehand, comprising: - a first component carrier for receiving a first type of preassembled structures (7, 7') - a loading unit which comprises a first manipulator, wherein the first preassembled structure is placed by the first manipulator onto the component carrier in the loading unit, - a joining unit, which comprises at least one joining robot (4c - 4f) with which the first preassembled structure is joined together, and - a transport unit by means of which the component carrier is movable between the loading unit and the joining unit, - wherein the transport unit transports the component carrier, which is equipped with the first preassembled structure into the joining unit and - wherein geo-connections of the first preassembled structure are produced in the joining unit by the joining robot or the joining robots (4c - 4f), - wherein the joining unit comprises a second manipulator, which lifts the preassembled structure off the component carrier after the joining of the geo-connections by the joining robot or the joining robots (4c - 4f), - wherein the transport unit conveys the component carrier back to the loading unit while the joining robot or the same joining robots (4c - 4f) produces or produce welded joining-connections - wherein for this purpose the structure stabilized by the geo-connections is lifted by the manipulator in the direction of the arrow (z) off the table of the joining station from a geo-level I to a welded joining-level II by a height H1 into a completed joining- position III, - wherein it is possible through a completed joining of the structure in a float position to remove the component carrier from the joining unit before the end of the joining operation and to load it again in the loading unit, - wherein the component carrier is equipped in the loading unit by the first manipulator with a second preassembled structure and - wherein the second manipulator unloads the completed joined structure from the manufacturing system (1, 10, 15, 18, 21, 22, 29) before the newly equipped component carrier is driven with the second preassembled structure into the joining unit. - a first component carrier for receiving a first type of preassembled structures (7, 7') - a loading unit which comprises a first manipulator, wherein the first preassembled structure is placed by the first manipulator onto the component carrier in the loading unit, - a joining unit, which comprises at least one joining robot (4c - 4f) with which the first preassembled structure is joined together, and - a transport unit by means of which the component carrier is movable between the loading unit and the joining unit, - wherein the transport unit transports the component carrier, which is equipped with the first preassembled structure into the joining unit and - wherein geo-connections of the first preassembled structure are produced in the joining unit by the joining robot or the joining robots (4c - 4f), - wherein the joining unit comprises a second manipulator, which lifts the preassembled structure off the component carrier after the joining of the geo-connections by the joining robot or the joining robots (4c - 4f), - wherein the transport unit conveys the component carrier back to the loading unit while the joining robot or the same joining robots (4c - 4f) produces or produce welded joining-connections - wherein for this purpose the structure stabilized by the geo-connections is lifted by the manipulator in the direction of the arrow (z) off the table of the joining station from a geo-level I to a welded joining-level II by a height H1 into a completed joining- position III, - wherein it is possible through a completed joining of the structure in a float position to remove the component carrier from the joining unit before the end of the joining operation and to load it again in the loading unit, - wherein the component carrier is equipped in the loading unit by the first manipulator with a second preassembled structure and - wherein the second manipulator unloads the completed joined structure from the manufacturing system (1, 10, 15, 18, 21, 22, 29) before the newly equipped component carrier is driven with the second preassembled structure into the joining unit.