I think we can all agree that the mail in ballots were all for Biden and that they "flipped the script". We aren't going to agree at all on what you believe motivated the mail in ballots. Dems saw opportunity, seized it, and ran it back for the score - it's just that simple. The other part that we've pretty much all acknowledged is that Trump is abysmally poor as a communicator, and this year perhaps more than any other (WW2 is questionable) needed an effective leader - and the key to that is someone who can communicate effectively to calm the masses. Despite rosy election maps on election night, all the evidence about how this election would turn was there - you just had to open your eyes. look, and for most of us watch the train wreck play out.
Everyone knew that the mail in ballots would favor Biden. Every pollster and expert in the business said that for the last 3 months, plus. And Trump knew it, too. That's why he went on the offense against them so much leading up to the election. Problem is, the system that elected him has also used mail in ballots. They just aren't that exotic. And the loser can always claim, with no evidence, that they were manipulated.
In this election, where states were allowed to count the mail in ballots as they came in, Biden started with a lead which dwindled and then was overtaken by election day voting by Trump. See e.g. Florida. But that is because Florida elections supervisors were allowed to count ballots as they came in.
Pa. was one of a number where Republican legislatures directed that no mail in ballots could even begin to be counted until November 3. So on that date, those states first tallied in person votes. And as in Florida, those skewed to Trump. Then, they turned their attention back to the mail in ballots, which take longer to count because they have to be opened and verified. So as those finally got counted, they went towards Biden, just as in Florida.
The two claims you hear are that 1) ballots got added after the fact. Not true. Or 2) ballots are improperly being counted for Biden. But in those counting rooms where there is any question you have a GOP person and a Dem person that review it and if need be go to a tie breaker person.
It would take a HUGE number of those ballots to really be in question so as to make any difference because there just aren't that many of them. We'll know exact numbers on that later, but it seems like a truly impossibly small chance of any result changing data there. So small as to be disregarded as we move forward with the transition to a new administration.