If anyone has a 2019 i9 iMac, if you could put it under 100% load, what is the temperature and fan speed of yours?
Short answer: CPU package 95C (with some cores up to 97) and fan speed between 1200 and ~2900 depending on other factors. This with i9-9900 and 580X 8GB GPU.
1. Usage of CPU and GPU both have effect on fan speed. I suspect it is a combination of many temperature readings.
2. Temperature is controlled by dropping the CPU speed as well as by raising the fan.
3. Though I have no concrete evidence I think that the settings for fan have changed since release a year ago. So it may be important to state macOS version when discussing these things. I started with 10.14.4+ and now run 10.15.3.
4. My fan only gets going when the CPU temperature is over 90.
5. When CPU bound but not GPU (using Prime95 stress test with 16 threads) the CPU speed comes down to a steady 3.9 GHz with CPU package temperature at 95C and only gently raises the fan speed. Note 3.9 is above the nominal 3.6GHZ for all cores 100%. 95C is the steady state temperature - the CPU is allowed to rise to 100C for a few seconds before GHZ drops and/or fan brings it down to 95.
6. Running the GPU hard (with e.g. Unigine Heaven) the fan speed will rise a bit, but only once the CPU temp has risen. The GPU can consume more power than the CPU.
7. Running both CPU and GPU hard rapidly lifts temperature to 95C, drops the GHZ to 3.9 and starts the fan racing to ~2900. I have never got the point where CPU drops below 3.9GHz.
8. Finally, I would not change from anything from system settings for fan control, unless very sure of what I was doing (I'm not).
In practice, I seldom notice the fans unless doing something both CPU and GPU intensive - in my case in Lightroom.
All I have said is just some different slant on what @casperes1996
has already said.