A lot of attention was given to the fact that Trump did better with Democratic leaning groups especially Hispanics, while Biden did better with Republican leaning groups like married men and evangelicals. I think that we should look at this beyond the demographic level and look at it this way: the blue got redder and vice versa.
You could see this same effect at the county and even state level. Five out of seven electoral units that saw two party vote swings towards Trump (California, Hawaii, Illinois, New York and Washington DC) had higher than average vote shares for Biden and thus Hillary Clinton (and all significantly higher than average at that), with the other two being considered swing states (Nevada and Florida). West Virginia and Wyoming, the two most Republican states in both 2016 and 2020, moved 5% and 7% towards Biden, more than twice the national shift.
Here is an image that shows the correlation between swing and 2020 vote share (it was -43%):
What caused the apparent depolarization in the 2020 presidential election? Was it higher turnout bringing out voters that vote in unexpected ways? People switching sides? Some combination or something else?