Hi,
Hope I am understanding your question correctly.
Nextcloud does have a CalDAV/CardDAV implementation accessible at the base URL endpoint: https://sub.example.com/remote.php/dav or
https://example.com/nextcloud/remote.php/dav
depending on your installation.
You can find some information in the Administration Manual, here, under Service Discovery:
https://docs.nextcloud.com/server/latest/admin_manual/issues/general_troubleshooting.html?highlight=service%20discovery#service-discovery
For anyone reading, do note the apache .htaccess template in the documentation is incomplete as of Nextcloud 20 at the time of writing, resulting in setup warnings for webfinger and nodeinfo. Visit this thread for more info.
Hope this helps.