This article explains how to install Nextcloud on the Raspberry Pi. It has been tested with the latest version of Nextcloud (16.0.3 at the time of writing), but should work for future versions too. It has also been tested on the latest Pi hardware, the Raspberry Pi 4, and on earlier Pi versions. Nextcloud is an open source software package providing remote file sharing services, similar to Dropbox. But with Nextcloud, you retain ownership, security and control of the shared data. Nextcloud works well on a Pi 2, Pi 3 and especially a Pi 4 but will run very slowly on a Pi 1.
Note: This is a manual, step-by-step procedure. If you would rather do the installation automatically, please see my recent article Automatic Nextcloud Installation on Raspberry Pi, which explains how to install Nextcloud with 3 commands. It is the quickest and easiest way to get Nextcloud running. Both procedures achieve the same overall result, however.