This is a good place to start - https://una.io/page/Start-Guide-General

When you compare UNA with, say, Joomla or Wordpress, consider that UNA is a framework (with data model similar to Drupal) with a collection of modules built for a community/multiuser site. These are provided as an integrated ecosystem. to build a social network with Joomla or WP you’d have to use most of the modules from various 3rd party developers, which may not sync correctly between each other or follow the core updates pace. We do it all together instead - same team doing core updates and all modules. There are still 3rd party modules, of course, but at least you have a good bunch to work off. 

There is a bit of work to do when you start, but you’ll see during configuration that this is actually necessary if you want your community to be unique and not just another cookie-cutter instance of a common CMS.