So, in county vote counting centers with 15 to 100 workers and half dozen observers in a number of battleground states, the bosses would stop the counting tell workers to shred the ballots, go home and don't tell anyone?If you think election fraud was at all likely, think about how it might be done.
Well, if I were orchestrating the whole shebazz, what I would do is have all the hotly contested states stop counting the votes at the same time while Trump was ahead, close the vote tallying to any possible onlookers overnight, and then open back up in the morning with all of them showing Biden as the leader.
But, hey, that's just me! If you have a better idea as to how to steal an election, just let me know, k?

Seriously, there is no way of rigging a presidential election with any degree of certainty. In this election there were 10 battleground states and any of them could turn out to be the keys to victory. So the first task is to determine which states to target and then which counties and finally how many votes to steal all before the election. Assuming your crystal ball is accurate, you must have people in many counting centers flipping votes without being observed or getting caught by safeguards and audits. All this would require dedicated, knowledgeable people that would be willing to risk a life in prison on a venture with a very low likelihood of success.