Still don't understand combinators and logic in this game

I have to concede to the fact that after hundreds of hours, I still don understand how Factorio handles logics with combinators and such, I just end up copying stuff but haven't been able to build something on my own, just becaus I still don't get how the signals work.

I have dabbled in programming before (logic gates, C++, Delphi, PHP) but I still can't seem to wrap my head around the logic of "programmin" in this game. I would like to come up with some own designs but I just don't get how it works.

Take the simple Set/Reset latching switch. It works as it should, but I still have no idea WHY it works and HOW the signals travel and are read. How am I supposed to deduce that the output of the AC should loop back to the input of the DC and that will enable the DC do let the pump run only when Light oil is between 20k and 5k? What parameters do I have to change if I want a different result?