Fairfax County - 2016
Clinton - 355K
Trump - 158K
Fairfax County - 2020
Clinton - 420K (18% increase)
Trump - 168K ( 6% increase)
Fairfax county experienced a population increase of roughly 3% between 2016 and 2020.
Maybe so, but that is on a net basis, and therefore doesn't account for how many people of one political stripe may have moved out, only to be replaced by someone of the other political stripe.
Given that NOVA is both very transient, and very liberal politically, it stands to reason that even if the net change in population was only 3%, that more conservatives left than moved there, and fewer liberals left than moved there.