You're right about the fact that once the ballot is completely separated from the envelope and/or secrecy sleeve and moved to a different pile or room, there's no way of tracking which voter the ballot came from.
But your statement above is incorrect. Or at least it is for the County in which I live, and for which I carefully watched the counting operation last Tues evening as a GOP poll watcher. Caveat: my County doesn't necessarily count everything the exact same way as every other county. (FYI it tilts strongly Dem, though the County Clerk is still GOP for now but that'll likely change in 2 yrs when her term's up, given our rapidly changing demographics. I sure hope the transparency & fairness don't change when a Dem takes over her job.)
What I saw was this: Vehicles arriving at the back entrance each had two people in them, one R and one D. After the ballots were arranged into neat stacks face forward and top edge up, they went thru this machine which read the code on the envelopes to record in a DB the name of each voter, and to compare the sig on the back side of the envelope with the sig in their records. It was incredible how fast it did its job. (I asked, and they said they bought it around 8 yrs ago, so it's not even the latest technology).
The vast majority were deemed a sig-match, and only then did they get sent to a room with 3 machines that slit open the envelopes and separated out the ballots.
Signatures that the machine wasn't able to verify as matching were kicked out to a different output tray. These were then taken to the manual sjg-match operation. This consisted of at least 20 teams of people, 2 per team, again 1 from each party. They looked at magnified images of the sig on the ballot & the sig in the DB that was a scan of what the voter signed when they registered. Only if both the R & the D team members agreed they matched, did the envelope get sent to the room that slit open the envelopes.
I should have said that the vehicles I mention above were carrying the sealed ballot boxes from various polling stations around county.
Thank you for such detailed reply. If all counties followed that procedure it would make a secure election.
However there are dozens of videos where republican observers were forced to be 10, 20 feet away. And unable to see ballots being opened and see the signatures etc.