Events are transferred in iCalendar format (.ics), contacts in vCard format (.vcf). Nextcloud, Radicale, SOGo and many mail server suites support both. iOS and macOS speak the protocols natively; on Android DAVx5 usually handles it, on Windows Thunderbird.
Clients find the right path through /.well-known/caldav and /.well-known/carddav, which must redirect to the DAV endpoint, for Nextcloud /remote.php/dav. SRV records also help with automatic setup.
Typical problems: the well-known redirects are missing behind a reverse proxy, clients on MFA accounts need an app password, and very large address books or calendars with thousands of recurring events slow down sync.
Example
curl -sI https://cloud.example.com/.well-known/caldav