• how come Biden scored a record low 16.7% of counties but a record high of votes in history?
That would be because the electoral college counts individual popular votes by state, and not votes by county. Biden succeeded in winning more urban and heavily populated cities, while Trump succeeded in winning more rural and less populated counties. The key word is populated areas where the individual vote succeeds any overall county or state simplified voting system.
• If the only motivation was just to vote President Trump out of office- how come such a vast number of Biden votes were cast after Nov3- risking forfeit or being thrown out?
This is also a big question and seems to have an informed bias behind it. First, no votes were "cast" after November 3rd. Votes were /counted/ after November 3rd for a variety of reasons.
1. It takes time to count all votes. It usually takes weeks to determine but sometimes it can be only one or two days, especially if the victory is a landside.
2. Mail-in votes were a huge controversy in the 2020. Many states elected to count these separately and days after November 3rd to quell any concerns of election fraud. (Naturally, that didn't work.)
3. There were ballots mailed-in that didn't come until days after the election. These still had to be counted but they were more heavily (and rightly) scrutinized for their validity. To my knowledge, there is no catalogue anywhere that shows how many of these "late" votes were counted or thrown out.
• how come this wasn't a nationwide trend but occurred in six populous counties in crucial swing states that stopped the count on Nov-3, assessed the election outcome then extended the deadlines until they flipped?
First of all, counting did not stop on November 3rd. It continued well into December, which is common for any election. Presidential or mid-term. Voting stopped after November 3rd, which is totally different from counting. An error Trump made frequently so it's understandable for your informed error to think votes stopped being counted on November 3rd.
The media's coverage naturally focused on the swing states as non-swing states had already assessed who'd won by each candidate reaching it's targeted majority count a day or two. While it took much longer to make this same assessment in swing states because, naturally, they were much closer.
A higher percentage of voters than usual voted by mail-in ballot because we were, and still are, in the middle of a pandemic.