Difference between packet switch and circuit switch.
In the switching center, with subsequent routing of the message determined by the address indicated in the header and implemented wholly in software. In circuit switching, actual "relays" somewhat like those perhaps still found in telephone central stations make the necessary sequence of connections to route the message to its intended destinations. In older telephone exchanges and perhaps even now, those relays were/are rather amazing electro-mechanical kludges, with motors and gearing pulling a kind of rotor up and down and around through cylindrical arrays of contacts until the right destination contact was reached. (I think the very term "kludge" has some historical connection with these systems. Perhaps it was a brand name at one time.) And before that (for the benefit of younger readers), telephone operators (usually young ladies) sat in front of arrays (literally hundreds) of telephone "jacks", with an array of corded telephone ...