I recently bought a TTR that had starter gear problems and was kickstart only. It was only used off road so had no need for lights etc.
The previous owner must have been quite knowledgeable about bike electrics because he managed to get a working TTR using only a very small part of the original loom as in the pic below - amazing!
Since this electrickery stuff is someting that can't bee actually seen, I tend to be rather in awe of it all.
I understant its analogy with plumbing but why it does it and how it does it is mostly beyond me.
I once rewired an old '55 Triumph twin following a diagram and using the existing dynamo, regulator and switches - that was easy compared with the TTR's apparent spaghetti loom. My Triumph had a magneto so I had no worries with sensors, senders, CDI, ignitors, zener diode, rectumfrier and things.