Nowadays, local Calendar and Address book servers become more and more popular to get away from US dependencies - especially in Germany. Nextcloud and ownCloud are very popular here.
It is possible to set up the link via cardav / caldav, but this is complicated as you need to specify the full URL to the remote. Unlike Apple ICal, the mobile EM Client is not able to auto-discover the path. In ICal I only need to define the Cloud Host without path, ICal auto-discover the path.
But NextCloud and ownCloud have hardcoded paths (https://host/remote.php/dav/principals/users/userid).
I know how to set up my NextCloud instance, but I am an IT expert. A noob would fail.
On my IPad with IOS Calendar I only need to enter the host, IOS auto discover the path.