A head to head comparison of the 2020 Primary gives us an idea of the break down of the voters.
Joe Biden got 49% (1,2 millions) of the vote to beat Donald Trump (1 millions votes) by 40% and Bernie Sanders (288k votes) by 11%.
Joe Biden wins 14 out of the 67 counties in the state of Pennsylvania. Donald Trump won more counties than Joe Biden, but did not get as many votes as he did. Bernie Sanders did not win any counties.
Bernie Sanders vs Joe Biden
Joe Biden vs Donald Trump
Donald Trump vs Joe Biden
In a head to head match up on just the total number of votes, Joe Biden (82%) beats Bernie Sanders (18%). Joe Biden (55%) beats Donald Trump (45%). Donald Trump (79%) beats Bernie Sanders (21%).
These 10 counties play an important role in these primary elections. Unlike in California, in Pennsylvania, top county is not an order of magnitude larger than the other counties. Even though a candidate, can get a sizable majority in a large county it is possible for the other candidate to catch up from the smaller counties.
Joe Biden won an advantage in Philadelphia over Donald Trump but not obtained enough votes in the other counties to beat him in the 2020 Primary election in the state of Pennsylvania. Donald Trump achieved a majority vote in other populous counties, securing him the victory over his opponents.
Related References: 2016 Philadelphia General Election Story, 2016 Philadelphia Primary Election Story