Nextcloud is a PHP-based server software to access, share and protect your files, calendars, contacts, communication & more at home and in your enterprise.
/remote.php/dav (for instance:
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 DAVdroid). If you're using two-factor authentication, generating a device-specific password for DAVdroid is obligatory.