A head to head comparison of the Illinois 2020 Primary gives us an idea of the break down of the voters.
Joe Biden received 986,661 (46%) votes beating Bernie Sanders who received 605,701 (28%) votes and Donald Trump who received 520,956 (24%) votes.
In the primary, Trump won 83 counties in the 3-way match up. Biden trailed behind Trump with 20 counties and Sanders won 1 county in Illinois Primary.
Bernie Sanders vs Joe Biden
Joe Biden vs Donald Trump
Donald Trump vs Bernie Sanders
In a head to head match up on just the total number of votes, Joe Biden (62%) beats Bernie Sanders (38%). Sanders (54%) beats Trump (46%). Also, Biden beats Trump with a 65% over his 35%.
These 10 counties play an important role in these primary elections. If a candidate, can get a sizable majority in the three larger counties (Cook, DuPage and Lake), it becomes very difficult to win majorities in the smaller counties with out voter out reach and increased voter participation in them.
Joe Biden's top 10 counties of votes came from the top 10 vote size counties. This gave him an upper hand over Donald Trump in the state of Illinois.
Related References: 2016 Illinois General Election Story, 2016 Illinois Primary Election Story