Nextcloud provides an number of APIs for client applications to talk to.
WebDAV is the main api for file related operations, it supports listing directories, downloading and uploading files, manipulating tags and favorites and more.
An overview of how to use the various WebDAV APIs can be found at WebDAV client APIs, additionally Nextcloud implements rfc5323 to allow searching the filesystem more information about how to use WebDAV search can be found at WebDAV Search.
The OCS API provides all information that is not available via the DAV endpoints. This contains endpoints for user data or sharing capabilities for example. See OCS API’s for more details.
Other OCS API documentations: