Soldering electronics and sweating copper pipe are totally different from each other. Different solder, different flux, different methods.
What? That's just silly. Bonding solder to copper is the SAME process, whether its wire or pipe. The solder binds with the surface molecules of the copper.
To do that, the copper molecules have to be excited by heat to make room for the solder molecules to bind. Wire or pipe makes NO difference to that fundamental.
The flux chosen can have two purposes, one to clean surface oxides/impurities to expose the base metal. And two, to shield the exposed metal from further oxidation during the heating/soldering process. There are many compositions of flux.
As for the solder, plumbing solder doesn't have flux in it.
Some do, some don't. For mass installations, it is easier to apply a paste or liquid flux prior to adding solder. But for repair work, flux core wire is available at nearly any hardware store. You can actually use rosin flux core for the copper pipe bond. However, in drinking water pipes, the residual flux can contaminate the water flowing. Not recommended for ingestion. Acid flux residue dissolves far more rapidly in potable water and flushes out much more quickly. No doubt there are others flux types available.
I visited a circuit board production line where they used acid core flux in the soldering process. Generally speaking, acid flux left on a circuit board will corrode over time when it sucks moisture out of the air and reactivates the acid. This is why Rosin core is so widely recommended. The visited facility knew their chemistry and provided a neutralizing bath and a cleaning solution to remove any remaining acid.
A petroleum based flux must be added to encourage the solder to flow in to the pipe. Plumbing flux used to be acid based. I don't know why the change. Solder for electronics has a rosin based flux inside the solder. You are correct in that in soldering pipe, you heat the pipe and then apply the solder. With electronics, heating the solder first reduces the affect of either melting the insulation off of wire, or damaging electronic components or traces on a circuit board from excess heat.
Heating the solder first, often results in a cold solder joint, when hand soldering. This is where the solder has encapsulated the wire but has not bonded with it. There is a mechanical friction bond but not a molecular bond. The molecular bond allows full electrical conductivity. Cold solder joints won't flow electrical current fully, and often get worse over time, as oxygen penetrates the separation layer and oxidizes the base metal.
Cold solder joints are not smooth and shiny in appearance. Their margins don't always taper to zero, but rather end abruptly. Saves insulation, perhaps, defeats the electrical connection goal.
I've witnessed wave soldering production lines where the entire circuit board with parts on it is essentially dipped in molten solder. There was always the person near the inspection table afterwards, hand soldering the voids or cold joints that occurred. Even with temperature and time-in-the-wave controls, not all the joints were guaranteed, particularly at the beginning of a production run. These boards were made with all new, un-oxidized parts. Not even close to what 40 year old exposed copper has in oxidation.
It's not impossible to solder with a big enough solder blob, and I have done it in special circumstances. But, definitely NOT the way to learn proper soldering techniques. IMO. I don't know of any professionals that would advocate such.
Regards,