Start off in Michigan and Detroit. Biden actually got about 1,000 fewer votes than Clinton in Detroit. Trump, meanwhile, got nearly 5,000 more votes. Given Trump received so few votes in Detroit, this was good enough for a 65% increase for Trump.
Then look at the counties surrounding Wayne County (where Detroit is located) as well as the places in Wayne outside of Detroit. Biden saw a 25% increase in his vote share, while Trump’s vote share increased by just 15%. That alone was worth a net of more than 120,000 votes for Biden’s margin over Trump compared with Clinton and 2016…
Finally, in Wisconsin, Trump’s problem wasn’t Milwaukee. While Biden did pick up votes in the city of Milwaukee compared with Clinton, it was a rather small amount (about 6,000). Trump snagged an additional 3,000 or so votes. Trump’s percentage increase (because he started at such a low baseline) in his vote total of 7% in the city of Milwaukee was actually double that of Biden’s over Clinton’s (3%).
The suburban counties and Milwaukee County outside the city of Milwaukee are where Biden advanced the ball over Trump. Biden’s percentage increase of the vote in these counties (25%) compared with Clinton more than doubled Trump’s (12%). That’s the opposite of what happened in the city of Milwaukee, where Trump’s percentage vote increase was larger than the one on the Democratic side.