With E.U. based websites, being compliant with the GDPR is a requirement. Googles promise with Google Analytics 4 and server side tracking is, that this setup will be compliant (as per their definition). After reading about it and seeing that both servers, the Google Tag Manager Preview and the Tagging server, can be set up in multiple environments outside of Google, I can see where Google is heading *).
The final question remaining unanswered to me is: Where is the database?
*) Yes, they are also trying to move around the first party cookie and DNS limitation imposed by nearly all current browsers as per the California Consumer Privacy Act (CCPA), GDPR and ePR requirement.
Please note that this question does not search for legally circumvention. Meaning "everything's allright" does not answer this question. It's just about data storage for analytics/ tracking data. As a bonus it seeks to answer if a custom database or a database in a webmaster/ dev/ sysadmin defined location can be set up. Thanks in advance.