Group Policies can also be set to disable Windows Update altogether or to use the settings available from Windows 8.1 and earlier. I have mine set to "Notify for download and notify for install" and additionally I have disabled the ability for Windows Update to initiate a reboot to complete installing updates. Similar Group policies exist for other aspects such as Telemetry.
The Insider program is mostly for individuals and developers. - Right?
That's a charitable way of putting it. Microsoft laid off a significant amount of their Testing workforce in 2014, shortly before they started the program. The tasks that Insiders are expected to perform are effectively volunteering for programmatic testing roles. Saying "I'm a Windows *Insider*" makes people think they sound like they are part of some exclusive club, certainly more than "I do volunteer work to improve a proprietary software product for a massive corporation".
Now if a person does not want the insider updates on one of his computers, how would he do it?
Is there some way you tell the update serve that one of yer PCs is for the beta testing ant the other needs to have a stable system?
By not doing anything. You have to explicitly configure the system(s) in question to receive insider preview builds and configure it for the fast or slow ring.
I have two systems running insider builds on the fast ring, and my main systems on the standard consumer releases. The former I have on systems that don't matter very much. I just ignore the feedback hub- If I encounter problems, I just laugh at it. They can hire people to test their own damned product, rather than trying to pay me in "good vibes" or whatever they seem to be doing with the insider program.