The voter has to request an absentee ballot, with signature match etc. Ballots in all those swing states had to be requested by the voter.Usually it involves skimming mail in ballots from known non-voters. Apartment buildings are good for this. Also, since most places now recycle paper, harvesters can dumpster dive far more easily, as the ballots aren't covered in food waste anymore.
Some states with only vote by mail, do mail out in mass ballots to registered voters, but they have secured their voting rolls....