A few days ago, the New York Times had an article called "Why Trump's approval ratings on the economy remain durable." In the first paragraph, it asked "Why does President Trump continue to get higher marks on economic issues in polls than his predecessors Barack Obama, George W. Bush and George H.W. Bush enjoyed when they stood for re-election?" Its answer focused on growing partisanship and the declining number of swing voters. That is definitely part of it, explaining why his approval ratings haven't varied much, but it doesn't explain why he gets higher approval ratings on the economy than on other issues. The most recent Gallup poll, which finished on August 12, asked about the economy, foreign affairs, education, race relations, policy towards China, the coronavirus, and crime, and his approval ratings were highest on the economy.
Gallup has asked about approval on various issues fifteen times during Trump's presidency. It's almost always included the economy and foreign affairs, has included immigration and trade about half the time, and other issues less often, for a total of 23 issues that have been included at least once. Trump's mean approval rating on the economy is 51%, which is the highest of all the issues. The graph shows change in ratings on the economy, foreign affairs, immigration, and trade policy.
His ratings on the economy seem less "durable"--that is, more variable--than his ratings on other issues. For example, his approval ratings on the economy gradually improved until early 2020, but dropped from 63% to 47% between January and June 2020. His ratings on foreign policy also declined between January and June, by only from 47% to 41%. Probably the reason is that people can observe the economy more directly, so ratings respond to real changes in conditions. For something like immigration policy, most people don't have firsthand experience, and there's more room for interpretation of the things they hear about in the news. As far as why his ratings on the economy are still relatively strong, I think it's because the economic collapse of the last few months didn't just happen--it has an obvious outside cause, the coronavirus epidemic. You could still hold Trump responsible for mishandling the epidemic, but ideology can affect judgments on that.
His overall ratings on different issues are also of interest. There is a tendency for all to rise and fall together, so I, adjust for the time when questions were asked. "unemployment" is the baseline, simply because it was the last one on the list:
economy 9.6
terrorism 9.3
crime 6.7
defense 5.5
Korea 4.4
gun 2.0
China 1.6
trade 0.8
unemployment 0.0
taxes -0.2
budget -0.2
immigration -0.8
coronavirus -1.2
education -1.5
media -1.5
foreign -1.7
Iran -2.4
environment -4.8
Russia -5.8
race -5.9
health -6.8
corruption -7.5
Syria -9.2
There are definite differences in his ratings in different areas. To a large extent, they are what you'd expect for a Republican: stronger on issues where people like "toughness" (terrorism, crime, defense), weaker on ones where people like "caring" (race relations, health care). Some of them seem to be distinct to Trump, like the low ratings on corruption and relations with Russia. The fairly low ratings on "relations with the media" are also of interest, since many of his supporters see that as a strength for him.
[Data from the Roper Center for Public Opinion Research]