My understanding is that they have records of mail-in ballots sent out and returned.
So in theory they can confirm with each voter if they mailed a ballot. They can also ask them if the ballot was harvested.
They can also find ineligible and double votes.
“My understanding is that they have records of mail-in ballots sent out”
They don’t. Millions of mail-in ballots were dumped all over the place with no way to identify the voter.
There are “clean” records of Absentee Ballots. Although they are sent and receive by mail, they not “mail-in” ballots.
Completely and totally different.
From my recollection the ballot paper in Florida had neither my name or any number on it. So there is no way to extract and nullify my vote for Biden.