Most new water heaters that I'm installing have the T&P and the nipples preinstalled.
I am replacing a water heater that developed a slight leak at the cold inlet. At first I thought it was the nipple, but after disassembling and reassembling the connections (used Rectorseal #5 on the threaded connections) I realized it was a leak on a weld.
So when installing the new one, do they come with the T&P already installed? If so, do I need to remove it, dope it, and screw it back in? Ditto with the nipples. And should I be worried about over tightening threaded connections?
Ah ok. So then I just leave them alone.
I'll attach the dielectric unions to the nipples and run copper from the unions up. That's pretty easy.
Depending on the one you buy, the nipples may already BE a dielectric union...no reason to add another one. Also, depending on where you live, some places REQUIRE the connection to be made with a flexible supply, and some PROHIBIT it. It's much easier to screw the flexible copper supply pipe in rather than try to align it with hard copper, but either can work, depending on what your inspector wants.
Important note - I'm not a pro
Retired Defense Industry Engineer