Re: how positive & negative plates are decided in a cylindrical electrolytical capaci
I don't know all the details but from what I understand, when electricity is applied at the proper polarity and intensity, it causes a region of electrolyte next to one of the plates to turn into a microscopically thin insulating barrier.
Once it is 'formed', the capacitor only works at the same polarity. Bad things happen if you apply reverse polarity.
I don't know how they manage to prevent the plates from short-circuiting.
I suspect manufactures have to keep a few things a 'trade secret' as they must try to make electrolytics as reliable as possible.
Electrolytics are prone to become 'leaky', or to increase in ESR, or to drop in Farad value. In spite of that, they are a bargain with their high capacitance packed into a small package.