Trump's improvement in (some) large blue counties

Very few counties changed hands in 2020 compared to previous elections. In Georgia, a turnover state, Trump was the only one to flip a county (the small county of Burke, GA). In Florida, even as Trump expanded his margin by over 2%, only Biden flipped any counties (and he flipped three, all casting over 100,000 votes). 

Nationally, the election was close (about as close as 1948 in the popular vote), and yet Trump flipped only 15 counties. All but two cast fewer than 20,000 votes. In contrast, McCain flipped 44 counties in 2008, of which at least four (Washington, PA, Beaver, PA, Fayette, PA, and Pike, KY) cast over 20,000 votes--despite that he was losing nationally by a decisive margin. At the same time, Trump failed to improve very much on his 2016 vote share in many of the counties attracted to Trump's candidacy in 2016. In Luzerne, PA, Macomb, MI, Hillsborough, NH, Niagara, NY, and Suffolk, NY (to take some examples of counties Trump flipped in 2016 after they had voted for Obama twice, all of which save Hillsborough, NH had also voted for Gore in 2000), for example, Trump's vote share actually declined in 2020.

Much of the explanation for how Trump kept it as close as he did in 2020 would seem to lie in his improvements in large, heavily blue counties. These counties were so blue that they weren't going to flip (in most cases; some were close in 2016 and two--Stanislaus and Rockland--came fairly surprisingly close to flipping in 2020).

Some major blue counties (counties casting over 100,000 votes) in which Biden improved over Hillary Clinton by less than 3.1% and Trump improved on his 2016 vote share by more than 0.8% (bolded counties are ones in which the Democratic margin shrank by more than 6%; italicised counties are ones in which the Democratic margin grew)

Alameda, CA
Los Angeles, CA
Monterey, CA
Orange, CA
San Bernardino, CA
San Francisco, CA
San Joaquin, CA
San Mateo, CA
Santa Clara, CA
Solano, CA
Stanislaus, CA
Broward, FL
Dade, FL
Hillsborough, FL
Orange, FL
Osceola, FL
Palm Beach, FL
Oahu, HI
Cook, IL
Lake, IN
Suffolk, MA
Wayne, MI
Clark, NV
Essex, NJ
Hudson, NJ
The Bronx, NY
Brooklyn, NY
Manhattan, NY
Queens, NY
Rockland, NY
Cuyahoga, OH
Philadelphia, PA
Providence, RI
Cameron, TX
El Paso, TX
Harris, TX
Hidalgo, TX

In contrast, some major blue counties in which Trump improved on his 2016 vote share by less than 0.8% (bolded counties are ones in which Trump's vote share fell in 2020)

Pima, AZ
Contra Costa, CA
Fresno, CA
Riverside, CA
Sacramento, CA
San Diego, CA
San Luis Obispo, CA
Santa Barbara, CA
Ventura, CA
Adams, CO
Arapahoe, CO
Boulder, CO
Denver, CO
Jefferson, CO
Larimer, CO
Fairfield, CT
Hartford, CT
DeKalb, GA
Fulton, GA
DuPage, IL
Marion, IN
Caddo, LA
East Baton Rouge, LA
Orleans, LA
Baltimore City, MD
Baltimore County, MD
Montgomery, MD
Prince George's, MD
Middlesex, MA
Norfolk, MA
Ingham, MI
Oakland, MI
Washtenaw, MI
Hennepin, MN
Ramsey, MN
Jackson, MO
St Louis City, MO
St Louis County, MO
Bergen, NJ
Bernalillo, NM
Erie, NY
Monroe, NY
Nassau, NY
Onondaga, NY
Westchester, NY
Forsyth, NC
Guilford, NC
Mecklenburg, NC
Wake, NC
Franklin, OH
Hamilton, OH
Multnomah, OR
Allegheny, PA
Bucks, PA
Chester, PA
Delaware, PA
Lackawanna, PA
Montgomery, PA
Davidson, TN
Shelby, TN
Bexar, TX
Dallas, TX
Fort Bend, TX
Travis, TX
Arlington, VA
Fairfax, VA
Henrico, VA
Loudoun, VA
Prince William, VA
Richmond City, VA
King, WA
Dane, WI
Milwaukee, WI

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There seems not to be a hard and fast rule about which kind of large blue counties Trump improved in, and which he did not. Almost all of the ones in which he really significantly shrank the Democratic margin were counties that had voted for Clinton twice and for Gore and Kerry--i.e., they were not formerly Republican counties that turned blue during the Obama years. (Osceola, FL, which voted Republican in every election from 1952 through 1992 and again in 2004, is an exception.) However, beyond that, many of the counties Trump improved in were former Republican strongholds, like Orange, Hillsborough, and Palm Beach, FL, Santa Clara, CA, and, to a lesser extent, Orange, CA and Harris, TX. On the other hand, many of them were, as one might expect (or at least find less unexpected), counties that had long been Democratic, amongst them many urban counties: San Francisco, Los Angeles County, Philadelphia County, Cook County (IL), Cuyahoga County, Alameda County, The Bronx, Brooklyn, Queens. 

Both kinds of counties are also found in the list of large blue counties in which Trump improved by less than his 0.8% national improvement: urban counties that have long been Democratic, many of them in the Rust Belt, such as Erie, NY, Milwaukee, WI, Lackawanna, PA, and Allegheny, PA; but also many 'ideopolis' counties (either formerly Republican or long Democratic), such as Dane, WI, Denver, Boulder, CO, Franklin, OH, Wake, NC, and Fairfax, VA.

It's hard to explain why Trump improved in San Francisco and Santa Clara but not in Denver, Boulder, or Multnomah County, or why he improved in Cuyahoga and Philadelphia Counties but went backwards in Allegheny, Milwaukee, and Erie (NY) Counties (in the last, he went backwards by 2.7% after having notched the highest vote share this century of any Republican in the county).