Question is straight forward enough: where does the loopback stack reside in linux, and to a lesser necessity, windows? In linux it appears to be a kernel driver, but I don't know where to look to find it in the kernel memory or source. It seems to be interconnected with everything due to the ties to all services.
The loopback interface is a driver, therefore it is found in the drivers subtree of the Linux source. See drivers/net/loopback.c.