Each state sets its own law for when absentee/mail-in ballots can be counted. For one example, Pennsylvania law saws that counting of ballots, both mail-in and in-person, starts when the polls close (8pm on Election Day). This year, citing extraordinary circumstances, the Republican-controlled state legislature there passed a measure allowing counties some flexibility, meaning they didn't have to start counting until the next day. In response, many counties then announced that they would have enough on their hands just counting the in-person vote, and wouldn't even attempt to count the mail-in vote until the next day. One might note that this allowed the president to go on national television and say that all counting should be stopped before it even began in many counties of one of the most consequential states in the election. One might also note that in-person vote trends Republican while mail-in trends Democrat. Then again, one might just be a Democrat pussy with Trump Derangement Syndrome.