Absentee Ballots are a gateway to voter fraud. That is one reason blue states like California no longer have polling places in some/most areas. Absentee ballots can be manufactured/found post election day. This stuff has got to stop! (Maybe the coming civil strife will take care of that problem most effectively.)
Early voting is the worst when it comes to fraud.
I know of one way absentee ballot vote fraud happens from the personal experience of a relative.
Democrats in local government election offices scour the data base for registered voters who do not usually vote.
They pass those absentee ballots to other democrats who fill them out and mail them in.
They do this early in the voting window because in case of a duplicate the first ballot is assumed to be the correct one.
If the real registered voter shows up to vote they have to submit a provisional ballot and a protest to challenge the first (false) ballot.
Then the supervisor of elections office must determine which ballot is authentic.
And remember that it is the democrats who control the local governments and elections offices in most cities.