A head to head comparison of the Massachusetts 2020 Primary gives us an idea of the break down of the voters.
Joe Biden received 473,861 (30%) votes to beat Bernie Sanders, who received 376,990 (24%) votes and Donald Trump, who received 239,115 (15%) votes.
In the Primary, Trump won 55 counties in the 3-way match up. Sanders trailed behind Trump winning 9 counties with Biden winning nothing in Colorado Primary.
Bernie Sanders vs Joe Biden
Joe Biden vs Donald Trump
Donald Trump vs Joe Biden
In the head to head match ups, Biden (56%) beat Sanders (44%). Sanders (61%) beat Trump (39%). Also, Biden (67%) beat Trump (34%).
These 10 counties play an important role in these primary elections. If a candidate can get a sizable majority in the larger counties (Boston, Cambridge and Newton), it becomes very difficult to win majorities in the smaller counties without voter outreach and increased voter participation in them.
Both Biden and Sanders were almost tied in votes from Boston (the largest county), beating Trump by 37k votes just in that county. Biden's steady win at many other important counties gave him the upper hand in the State of Massachusetts.
Related References: 2016 Massachusetts General Election Story, 2016 Massachusetts Primary Election Story