Mail in votes are counted after day-of-election votes. I have no idea why...that's just the way it is. Actually, I have an inkling: it they were counted first, the results would likely be leaked, which could either discourage day-of voting or encourage fraud.
You can Trump an idiot all you want for discouraging mail-in voting, but you should at least mention the context so that his suggestion makes sense: he felt mail-in voting wasn't secure and that votes cast for him would be tossed. That may have actually happened. Guess what? If it did, it jibes with the democrats rebuttal to the claim that 2020 was plagued with fraud because "there is no credible evidence of fraud." A destroyed ballot is destroyed, and hence isn't evidence.
As far as Pennsylvania goes, there was some scandal and it was litigated in courts. Dem counties received instructions that they could reach out to voters who has incomplete ballots and complete information that would spoiled the ballots otherwise. Republican counties did *not* receive these instructions. The case rose to the level of the Supreme Court, where the argument used was equal protection (equal application of the law guaranteed by the Constitution) had been violated. Ironically, the case was dismissed because the GOP waited too long to file it:
So where does that leave us for 2024? Well, Dems are fantastic at ballot harvesting and use it everywhere it is legal to get votes from those who don't want to go to polling places to cast their vote. Republicans need to catch up. Expect a knock-down, drag-out fight over mail-in voting (justified in 2020 due to Covid, to be argued over endlessly in 2024 in the absence of a pandemic).
Still think Biden gets re-elected. But of interest, the the only previous President to serve non-consecutive terms (Grover Cleveland) got more votes in the election he lost than the first one he won. Trump joined him in that category in 2020.