Democrats tend to do best in large urban areas, and there was significantly more voter turnout in 2020 than for Obama's election due to the drive. In 2008 voter turnout was just 57.1% while in 2020 it was 62%. Both are dismal numbers for a democracy, especially when elections have been close. You can blame part of the problem here to the fact that a lot of people just stayed home.
Also it's not the votes per county that matter. It's the votes per congressional district for the House part of the electoral college and votes by state for the Senate part of the electoral college. Furthermore, the votes in pretty much every state are aggregated and the winner gets ALL of the state's electoral votes. So even if a candidate only got 51% of that congressional district/state's vote, that person gets all the electoral votes for that district/state. That is why we've seen both Democrats and Republicans get a majority of the popular vote but don't win -- they lose in the electoral college vote and using that system not everyone's vote is equal.
The electoral college system is required by the Constitution, no vote rigging needed. Those results are why I think that it's long past time to ditch the electoral college and simply go by the actual number of votes cast by each candidate.