Hello Bernd,
the system-apache acts as a reverse proxy and passes the site-specific requests to the site-apache. You can use the system-apache also for other stuff, it will only proxy the requests for the /<SITENAME>-paths to the site-apache. The system-apache listens on port 80 and the site-apache listens on port 5000 (by default).
For how the system-apache proxies the requests there is somewhat of a “config-file-chain”.
Inside the config-directory for your system-apache there should be a file called zzz_omd.conf, at least that’s what its called on Debian-based systems. This file just tells the system-apache to include all config files in the directory /omd/apache/. There you will find a config file for every site you have on that server. These files in turn link to the config-files inside your sites, for exaple /omd/sites/SITE/etc/apache/mode.conf. In that file the config file /omd/sites/SITE/etc/apache/proxy-port.conf is included and there you will finally find the proxy-directives for your site.
The last file is also automatically created and updated when you for example change the site-apache-port from 5000 to anything else. That is how the system-apache then knows where to reach your site.
Hope that helps!
Cheers,
Lorenz