I have been fascinated by this too. I did some research and found this article written about how classic slot machine works.on How stuff work website. The machine engineers were pretty detailed aand we have to appreciate the effort that went into thos wheels, levels and gears that made up the old slot machine.

I know you'll be interested in this article so here is the link - https://entertainment.howstuffworks.com/slot-machine.htm?srch_tag=gw5f3ujklwwk7u3hmespyuf4c77ou7jm

I'm terms of manipulation, well I wouldn't call it that. I'll say house edge because it has to be set in such a way that the operators has more wins so as not to go bankrupt. No manipulation just some mechanical settings.