In Drupal 8, Search API Solr is the consolidated successor to both the Apache Solr Search and Search API Solr modules in Drupal 7. I thought I'd document the process of setting up the module on a Drupal 8 site, connecting to an Apache Solr search server, and configuring a search index and search results page with search facets, since the process has changed slightly from Drupal 7.
Install the Drupal modules
In Drupal 8, since Composer is now a de-facto standard for including external PHP libraries, the Search API Solr module doesn't actually include the Solarium code in the module's repository. So you can't just download the module off Drupal.org, drag it into your codebase, and enable it. You have to first ensure all the module's dependencies are installed via Composer. There are two ways that I recommend for doing this (both are documented in the module's issue: Keep Solarium managed via composer and improve documentation):