What app can I use to monitor iMac operating temperatures?

I use MacFanControl to monitor the Temp. Sensors. see > https://crystalidea.com/macs-fan-control

I use the free Temp Monitor to monitor over 25 temperature sensors in the iMac and it also can control the fan speed if needed:

Please Google the Technical Specifications for the model year iMac you are inquiring about and that will quote its operating temp range. For example if you have a 2021 24" iMac simply Google "2021 24" iMac Technical Specifications".

IF you are attempting to ask something else, please re-write your question so it is more clear. Saying "Apple to Monitor iMac Desktop operating temperatures." makes very little sense!

That is very nice, now to find out the operating temps please follow the instructions I provided in my initial post.