I disassembled (tore apart?) an old 812H-1C-C relay to see what's inside, and found an expected piece of metal which moves between two other pieces of metal when activated by an electromagnet.
I was however surprised that there is nothing special to prevent arcing, and the contacts were rather small, compared to the contacts of a 10A 250VAC wall socket, for instance. Despite the fact that the relay was used a lot, there was no traces of contact wear.
What's the trick? Why arcing is such an issue with wall sockets or any other connector, but is not an issue inside a relay rated 16A 277VAC? Is it because the relay is sealed, and filled with non-conductive gas which somehow prevents an electrical arc?