It is up to the app's developer's how the app utilizes the CPU cores. Some workflows are not conducive to utilizing multiple cores. So at times an app may only be able to utilize a single core, while another workload in the same app may be able to utilize all the CPU cores. Make sure macOS is up to date and all your third party apps as well. Make sure you are using the correct versions of third party apps which are compatible with the OS you are running (check the developer's website to make sure you using the correct version of the app(s) for your OS).
It is not unusual for an Apple laptop's temperatures to be in the 60C to 80C range even for "light" tasks.
It is also possible you have some other third party software installed that is causing the computer to run hotter. Typically anti-virus apps, cleaning apps, and third party security software will interfere with the normal operation of macOS including making the Mac run hotter than normal.