Sync contacts between Nextcloud and countless other apps

Nextcloud is free software for storing data (e.g. files, calendar, contacts, etc.) on a server. The user can access the data both via a web interface and with client applications (smartphone and desktop). Server and clients can synchronize themselves. Nextcloud enables the user to access a central and consistent database from many end devices and optionally share it with other users. In addition to data storage, Nextcloud offers functionalities for video conferences and various office applications via the web interface.