Apparently Cal made it legal to bring in boxes of ballots after the election. They just gave it a snappy name and everyone approved. No Rep ever fights back so it will never change.
In California, anyone can request an absentee ballot (no reason needed). Those ballots are valid if postmarked before midnight on election day and received by the following Friday. It takes a long time to count those mail ballots because each is cross-checked to make sure the same voter didn't also vote in person on election day.
The people who vote at the polls on election day tend to skew older and more conservative than the people who mail in their ballots, and younger voters are the most likely to mail their ballots at the last minute.