Which information exactly do you mean?
Most is from the aerial imagery.

The voltage tags are inferred from connecting lines. They are additionally verified via company documents & reports and government reports (for example at https://elibrary.ferc.gov/eLibrary/search, FERC Form 1 Substations)

operator is similar. If I know the general operating area of an utility (example https://www.swepco.com/lib/docs/company/about/SWEPCOServiceTerritory_Cities2018.pdf), and public records at FERC indicate presence of substations/lines by that company, they're tagged as part of that company. 
Also a very important aspect is just general (local) knowledge about how the electric transmission grid works. With the knowledge I have today, there's actually a lot of 'old'/my first power/electricity mapping of mine in Georgia, which I had (&still have) to clean up, by the way.

The reason this was uploaded with JOSM was simply because there were unresolveable conflicts while I tried to upload with iD (related to osm.org/note/3726315), so I downloaded my changes into JOSM to resolve the conflicts/upload from there.

Hope this answers the questions.