Democrats could only cheat in districts they controlled tightly.
In those districts, every registered voter submitted a ballot whether they wanted one or not.
Those ballots voted for Biden and the statewide offices (Senate races)
That is why the GOP did so well in congressional races, and state houses, but did poorly at the state level and national level.
It’s much easier to do that for them - their districts are more tightly populated - a single car going around collecting ballots can reach a lot more people in a much smaller geographic space. A GOP car would have to travel hundreds of miles to do the same.