shared calendars are not present in CalDAV
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I think each calendar has a separate CalDAV URL . So, there is no way to discover other calendars via CalDAV. But I am not an expert here. Maybe https://community.mailcow.email/d/1538-shared-calendar-android-phone helps.
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https://www.mail-archive.com/users%40sogo.nu/msg33275.html is the thread for future viewers.