A head to head comparison of the Oregon 2020 Primary gives us an idea of the break down of the voters.
Joe Biden received 403,594 (46%) votes to beat Donald Trump, who received 356,986 (40%) votes and Bernie Sanders, who received 124,559 (14%) votes.
In the Primary, Trump won 28 counties in the 3-way match up. Biden trailed behind Trump winning 8 counties with Sanders winning nothing in Oregon Primary.
Bernie Sanders vs Joe Biden
Joe Biden vs Donald Trump
Donald Trump vs Joe Biden
In the head to head match ups, Biden (76%) beat Sanders (24%). Trump (74%) beat Sanders (26%). Also, Biden (53%) beat Trump (47%).
These 10 counties play an important role in these primary elections. If a candidate can achieve a sizable majority in Multnomah, Washington and Clackamas (the largest counties 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.
Multnomah and Washington, the two largest counties, voted in favor of Biden giving him over 87k vote advantage over Trump and over 99k vote advantage over Sanders. Even though Biden won less counties than Trump, his steady win at the largest counties gave him the upper hand in the State of Oregon.
Related References: 2016 Oregon General Election Story, 2016 Oregon Primary Election Story