A head to head comparison of the Virginia 2020 Primary gives us an idea of the breakdown of the voters.
Joe Biden received 705,501 (47%) votes to beat Bernie Sanders, who received 306,338 (21%) votes and Donald Trump, who received 198,691 (13%) votes.
In the Primary, Biden won 130 counties in the 3-way match up. Sanders trailed behind Biden winning 3 counties with Trump winning nothing in Virginia Primary.
Bernie Sanders vs Joe Biden
Joe Biden vs Donald Trump
Donald Trump vs Joe Biden
In the head to head match ups, Biden (70%) beat Sanders (30%). Sanders (62%) beat Trump (39%). Also, Biden (78%) beat Trump (22%).
These 10 counties play an important role in these primary elections. If a candidate can achieve a sizable majority in Fairfax (the largest county by voter population) they can secure the win. It becomes very difficult to win a majority of votes in the smaller counties and come out ahead.
Two of the larger counties (Fairfax and Henrico) voted in favor of Biden, giving him an advantage of 87k votes over Sanders. Trump recieved very few votes from the larger counties. Biden's steady win at most counties gave him the upper hand in the State of Virginia.