DAVx⁵
Nextcloud is a PHP-based server software to access, share and protect your files, calendars, contacts, communication & more at home and in your enterprise.
DAVx⁵ settings
Base URL:/remote.php/dav (for instance: https://example.com/nextcloud/remote.php/dav)
Contact group method: groups are per-contact categories (other methods are supported on server side, but the Contacts Nextcloud app only uses CATEGORIES at the moment)
User name and password: same as in Nextcloud (or better, a specific device password for DAVx⁵). If you're using two-factor authentication, generating a device-specific password for DAVx⁵ is mandatory. If you are using two-factor authentication for your Nextcloud account, device-specific passwords are the only way to configure clients.