“but nobody ever explains exactly what they did.”
They exposed all of the cheating and criminal activities that DNC and Hillary campaign was doing. Apparently, that is how the Russians supposedly interfered.
The problem is that we do not know that. The DNC did not allow the FBI to examine their servers. I believe a Google owned company called Crowd Strike is the entity that examined the servers and said it was the Russians. Google was all in for the PIAPS so I do not believe them. I strongly suspect that the DNC was not hacked by anyone. Julian Assange said it was not the Russians that gave him the information and I do believe him. I think when Assange offered a reward to solve the Seth Rich murder, he gave away his source.