The registrar of voters in California counties should be releasing the first results from the mail-in ballots returned before today. I received a phone call and a text from the Secretary of State saying my ballot, which I dropped off on Saturday, was processed and accepted.
I used to work the polls as a precinct inspector, the one in charge of an individual polling site, and although the polls close at 8 pm it took us about 45 minutes to afterwards to pack everything up and do the paperwork before we delivered everything to a consolidate pickup location. Not every polling site turned in their materials at the same time but usually by 9:30 pm the ballots and voting materials/equipment will be on their way to the Registrar of Voters. There, it will take some time for the ballot to be scanned/tabulated and the results released. Those would be processed around 11 or 12 pm.
Mail-in ballots dropped off at a polling site or at a dropoff box, the sealed envelope containing the ballot is scanned by a machine comparing it to the signature on the computer stored registered voter form. The ballots then have to be opened and run through the scanning/tabulating machine and that can take a couple of days. For mail-on ballots received after Tuesday, the same process will be used. Note, for ballots dropped off at the polling site, ballot dropoff box, or received in the mail on/after election day will be compared to the signed voter lists at the polls to ensure no one has voted twice.