There will be more mail-in votes than ever before in 2020. Those who were uncomfortable with large gatherings often chose to vote by mail. Those who weren’t uncomfortable with them were more likely to vote in person. Also, in this election, Democratic candidates strongly encouraged people to vote by mail, while Republican candidates discouraged it. So it’s very likely that a majority of mail ballots will be from Democratic voters. These ballots may take several days after November 3 to validate and count.
The long time it takes to count mail-in ballots may make it seem like Democratic candidates are suddenly doing better in the days after the election. The votes were already cast, all that’s changing is how many were counted. Counting all valid ballots is required for a free and fair election.