re: Pollster says Trump wins election night, but Biden wins the White House a week later

You don’t understand the system. The popular vote isn’t for the president but for the slate of electors pledged to a candidate. Just because a network calls a state, based upon exit polls doesn’t mean that the state is official. It’s not official until the election officials in the states certify the winner. The press could project a winner, but if a cache of ballots is found, the result could change based on the new ballots. This could happen in MI, WI, PA, FL, NC among others.