trackingServer is important in two ways:
- That you're sending data to the right location
- That visitor ID cookies are stored in the right place
I'd recommend reading this article, which helps you understand how to populate trackingServer: https://marketing.adobe.com/resources/help/kb/en_US/analytics/kb/determining-data-center.html
If you're implementing code for Jan Exner's site, all of those ultimately end up at the same place. Personally I'd prefer using the first party domain if you're implementing code on jan-exner.de.
However if those are domains you got from like a blog post or something, then those are definitely not the domains you want to use. Instead, the above KB article will tell you what it is best defined as. Consistency is most important, as visitors under different trackingServers are treated as different unique visitors, even if they're on the same computer.