Welcome!
Most e-pipes take an 18350 battery. There are some that take 18490, 18650, and even 26650 (I think that's the measurement). For brand, AW IMR's or eFest flat tops seem to be what most people use.
The key ring is what is attached to the negative wire, and it is set in the perimeter of the bowl of the pipe, just above the level of the battery when it's installed. In a pipe with an ePipeMods-style switch (magnetic button that is attached to the negative end of the battery), when depressed, the metal edge of the button comes in contact with the key ring, and it completes the circuit to fire the mod. I hope that makes sense.
Thanks so much for the explanation...I've decided I'm going to build me an e-pipe. I semi built one over the winter but the wood cracked and I picked up analogs again. Now I'm getting back into the vaping world and want to pick up my e-pipe fetish again. I'll post pics once I get underway with it