It's from their "Voter Analysis" based on surveys conducted by NORC at the University of Chicago, an independent research firm.
The Fox News Voter Analysis, conducted in partnership with the
Associated Press, provides a comprehensive look at voting behavior,
opinions and preferences as America votes. It is based on surveys of
the American electorate conducted by NORC at the University of
Chicago. For the 2020 Democratic presidential primaries and caucuses,
the FNVA results are based on interviews with a random sample of
registered voters drawn from state voter files. In select states,
these probability sample results are supplemented by interviews with
self-identified registered voters recruited from nonprobability online
panels. The FNVA methodology captures last-minute voter trends, which
are of particular importance in the early caucuses and primaries, as
well as the sentiments and behavior of early and absentee voters, who
are significant on Super Tuesday and other later primaries.