By default, org-caldav does two-way syncing, that means it does not
matter where and how you change an entry. You can also move Org
entries freely from one file to another, as long as they are all
listed in org-caldav-files.
To do one-way sync only, set org-caldav-sync-direction to
'org->cal or 'cal->org, depending on which direction you’d like to
have. If you choose 'org->cal, then org-caldav-inbox won’t matter
and can be nil. Likewise, if you choose 'cal->org, then
org-caldav-files will be ignored and only the calendar will be
imported into the inbox.
WARNING: It is NOT safe to switch the same calendar between 1-way and 2-way sync modes. Doing so may cause unexpected behavior, such as deleting all events in the calendar.