TBH, what I know and how I've learnt it, has been very much from "paint by numbers" electronics. (connect wire A to lug X, now connect wire B... you get the idea )
The volume of electronic theory I possess would struggle to fill a thimble, but I manage to make things work and troubleshoot when needed.
I have just basically taken it on faith that the middle lug was always output, can't necessarily tell you why.
Also, we should be clear on how we refer to the lug numbers. TTBOMK, this is the accepted distinction: