A head to head comparison of the Alabama 2020 Primary gives us an idea of the break down of the voters.
Donald Trump received 695,469 (61%) votes beating Joe Biden who received 286,065 (25%) and Bernie Sanders who gained 74,755 votes (6%).
Donald Trump won 56 counties whereas Joe Biden won 11 counties in the total of the 67 counties in the State of Alabama
In a head to head match up on just the total number of votes, Joe Biden (79%) beats Bernie Sanders (21%). Donald Trump (90%) beats Bernie Sanders (10%). Also, Trump (71%) beats Joe Biden (29%).
Bernie Sanders vs Joe Biden
Joe Biden vs Donald Trump
Donald Trump vs Bernie Sanders
These 10 counties play an important role in these primary elections. Even if a candidate does not achieve a sizable majority in one of the larger counties (Jefferson, Mobile), they can still take the lead by capturing very large minorities in several counties.
Although Joe Biden won in Jefferson and Mobile, he could not achieve a victory since Donald Trump captured so many voters across most counties. This gave Donald Trump an upper hand in the state of Alabama.