I understand what you are writing, but one factor not accounted for is Trump himself. He is an incredibly polarizing candidate. Probably more so than anyone ever. He brought out his own voters and voters against him. I think 2020 was all about Trump. No one was going to the polls because they wanted Biden as president. They were going to vote for or against Donald Trump.
But only in the top population counties in all the states.
In the lower population counties, there was less enthusiasm for Short Bus Biden than there was for Obama in 2008.
Because those counties didn't matter to the overall scheme of the DNC IT guys.
I'll post up your South Carolina vote fraud command and control centers if you haven't done the county map thingee that I talked about.
You can see them in both 2016 and 2020 - in 2020 they're clearly delineated.
Florence-Darlington seem to be entwined, and I'm not sure what's going on with Horry County - BIG "turnout" there, but I've discovered cross-border "assistance" from neighboring states in other areas of the country [for example, a couple of eastern counties in Pennsylvania were "assisted" by New Jersey cross-border vote-riggers].
There are not enough good data points to put your finger on what's going on and say "AHA!!!".
The other key is that the scheme is obscured by the numbers.
You get state vote totals by the state only, or by county, and the county numbers are listed by alpha, mixed in with the superfluous "oaters" [other parties - Libertarian, etc.].
I didn't see the population component of the scheme until much later.
Once I saw it, it was easy to see in state after state.
Rigging Pennsylvania: Obama 2008 by county compared with Biden 2020 by county
https://freerepublic.com/focus/bloggers/4063855/posts
Rigging Georgia: Obama 2008 by county compared with Biden 2020 by county
https://freerepublic.com/focus/bloggers/4066008/posts