Nextcloud provides an official Nextcloud Android client, which gives its users access to their files on their Nextcloud. It also includes functionality like automatically uploading pictures and videos to Nextcloud.
For third party application developers, Nextcloud offers the Nextcloud Android library under the MIT license.
Android Nextcloud client development
If you are interested in working on the Nextcloud Android client, you can find the source code in GitHub. The setup and process of contribution is documented here.
You might want to start with doing one or two good first issues to get into the code and note our Prologue.
For authentication, you can use our usual login flow, and in addition (or instead if you are OK for users to depend on our Files app) use the great Android SingeSignOn library
Nextcloud Android library
This document will describe how to the use Nextcloud Android Library. The Nextcloud Android Library allows a developer to communicate with any Nextcloud server; among the features included are file synchronization, upload and download of files, delete or rename files and folders, etc.
This library may be added to a project and seamlessly integrates any application with Nextcloud.
The tool needed is any IDE for Android; the preferred IDE at the moment is Android Studio.