Why did Hillary do so well in Florida?

[First1There]

She actually put a lot of effort into turning out voters in Florida's urban areas (especially South Florida), and she was also able to leverage her husband's ties to the state to her advantage. She did better than Obama in the urban areas of the state; it's the suburban/exurban areas (especially in and around I-4) that gave the state to Trump.

Would she have won the state without the Comey letter?

You could argue either way. Exit polls showed that 11% of FL's electorate decided their votes in the final week before Election Day, and Trump won this group 55-38. On the other hand, I've heard political commentators say that Comey was at best "one of the factors" in late-breaking voters breaking for Trump.

Hillary spent a lot of money in Ohio. Lots of campaign visits too. 2016 was the last 2000s era election where the key states were seen as Ohio and Florida.

She actually put a lot of effort into turning out voters in Florida's urban areas (especially South Florida), and she was also able to leverage her husband's ties to the state to her advantage. She did better than Obama in the urban areas of the state; it's the suburban/exurban areas (especially in and around I-4) that gave the state to Trump.

I remember watching CNN after the 2020 election. President Biden made enough gains in Florida's suburbs to have won the state had he matched Hillary's performance everywhere else. But Trump's unexpected gains more than cancelled them out, unlike the other swing states.

Florida is probably the one state where it being highly urban hurt Democrats in 2020 lol

Actually, in the suburban/exurban counties in the I-4 region (namely, Brevard, Citrus, Flagler, Hardee, Hernando, Highlands, Lake, Manatee, Marion, Pasco, Polk, Sarasota, Sumter, and Volusia Counties), Trump improved on his margins in terms of raw votes in 2020 compared to 2016 (Trump gained about 232K raw votes in these counties in 2016 and another 45K raw votes here in 2020). Thus, Biden failed to make a significant dent in Trump's margins in this region, which was another contributing factor to Trump's improved margins in the state (in fact, if margins in the suburban/exurban counties listed above had reverted to 2012 levels, Trump's 2020 margin of victory in FL would have been a bit narrower than his 2016 margin of victory).

Where did Biden improve on Hillary in Florida and where did he decline?

https://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Florida_County_Swing_2020.svg/1920px-Florida_County_Swing_2020.svg.png

He improved in these places. This is a percentage swing map, so absolute vote gains might be different.