A welder is made up of many components. These various components and systems are a force system, a power system, control system, conductors, electrodes and cooling system. All play a vital role in delivering the proper pressure and current for a length of time to the parts to be welded.
This sketch shows the major components of a press welder. These same components in similar form are present in robot transguns and all automation weld systems. For instance, this shows an air cylinder where many transguns use servos for force application. In some cases transguns do not have tap switches on their transformers. Robots always use electrode caps and gun arms for holders not barrel holders as shown here. The point is the tools may be a different design but they perform the same function.
Reference: RWMA - Resistance Welding Manual 4th Edition