We are using the Drupal 8 Solr Search module, but originally this module only contains a simple search form. We would like to have an advanced search form, but we don't know how and where to begin to implement a custom one. Some sources recommended using Solr Facets but this is not enough for us.
I found a way without programming anything. In the Solr Search config page (Structure / Views) you can use the Views exposed filter to add as many index specific form input as you want.