The helper app is downloaded and installed outside of the Mac App Store. I've actually managed to find the working solution. XPC does work between apps within the same app group. The issue I had with configuring the mach service name. It turned out that it had to be in this format "team_id.group.your_app_domain.suffix", where suffix in my case was just "Helper".

Helper basically is another app as a LaunchAgent.

Thanks for the answer, @eskimo! In a non-sandbox environment, there is a way to fetch processor temperature, and there are many example apps doing that already. I must build it because that is the number one missing feature based on many users' feedback. In my app, as I see it right now, that would be an additional feature tough, that is available after installing the helper tool outside of the App Store. And there are two things I'm trying to figure out: Can I install an external helper tool for the app that is installed via the App Store? How can I establish communication between the main app (from the App Store) and the helper tool (installed from my website)?