1. For Ms outlook 2013 install plugin caldev
https://www.symplicity.com/wp-content/uploads/csm-how-to-caldav-outlook-synchronizer.pdf
configure:
https://www.webdavsystem.com/server/access/caldav/ms_outlook_caldav_synchronizer/
https://www.one.com/en/support/guide/setting-up-outlook-with-caldav-synchronizer#step-1
2. For Thunderbird install plugin caldev
https://www.zensoftware.co.uk/kb/Knowledgebase/How-to-synchronize-calendars-via-CalDAV-in-Mozilla-Thunderbird
3.
