I’ve always known these as reverse threaded coupling. Most people are aware of a union that couples two pipes together with a large nut. Unions are not allowed by code in some instances. Like in the image below, this is a tap to run a gas line underground to a shop.
The top arrow points at the reversed coupling. It gets screwed onto the pipe below and then you insert the nipple above and start wrenching it into place.
Because the threads at the top of the coupling and the threads at the bottom of the nipple are reversed you simply tighten the nipple into the top tee and it sucks it all together.
These couplings are available on Amazon in various sizes from