They don't. That also isn't usually what people talk about when they refer to Russian Hacking. Some of them may be thinking that, but so far the only such evidence of something like that being attempted was an attempt made by HOMELAND SECURITY (not Russia) to hack some of the more modern voting machines.
The machines should be not connected to the internet, but WHO GET'S THE CONTRACT TO MAKE THEM? I think it'd be pretty cool to do voting on a public blockchain that anyone could audit.
Yeah, I think my beef with the people insisting Russia hacked the election don't use common sense to ask question about how they could have done it. If they did they would realize they couldn't have done it and this would be done.
And if I'm not mistaken there is proof Hillary was involved with rigging the election but we don't hear a peep about that. It's all smoke and mirrors anyway.