Hi all,
@nicooo @Gorom @muck @tcit
be aware that sharing like this does not include encryption nor authentication.
So you basically share plain text the calendars (nextcloud and google) to everyone who got the link somehow and every knot on the internet can read it too.
It is meant for public calendars (like concert events).
A secure solution is to make a google / apple account and use for example outlook and caldav to sync between the instances. This is not a superb solution but the only I am aware of which offers security and authentication. If anyone is interested in a more detailed description drop a note.