The right-side of the pipe seems to be simply threaded into the T&P discharge valve with some plumber's tape, but the left side is connected to an (otherwise soldered) series of copper pipes by this nut looking thing. I'm curious how this arrangement was likely created, given the fact that these pipes are obviously not flexible connectors. Was it likely that the left side was connected first, and then the water heater was positioned into place exactly so the threading on the right worked? Or was this horizontal copper pipe screwed into the T&P release valve first (allowing the water heater to be positioned semi-independently) and then that nut-like thing somehow helping the copper piping to be connected to the elbow in a way which helped the fact that otherwise the piping would have to have been seemingly cut to the exact perfect length to work. Perhaps the nut allows for some fitting to be placed there and then the horizontal tube was soldered into place while connected to the water heater?
I don't need to re-engineer this or do something similar, but I'm curious how this connection would have likely been made with rigid piping. What is this nut? How would the left-hand side likely been connected?