When you choose to display your search results with the ajax search theme delivered with the plugin, several things happen.
Search page
Wpsolr creates a search page “search-wpsolr”, which only content is a WordPress shortcode. See Is there a shortcode to display the ajax search where I need ?
Search form(s)
The standard WordPress filter ‘get_search_form’ is intercepted to replace your theme’s default search boxes with wpsolr’s search box.
<a href="https://developer.wordpress.org/reference/functions/add_filter/" target="_blank" rel="noopener noreferrer">add_filter</a>(<a href="https://developer.wordpress.org/reference/functions/get_search_form/" target="_blank" rel="noopener noreferrer">'get_search_form'</a>, 'solr_search_form' );The wpsolr’s search box form’s target url is then set to to the new page “search-wpsolr”.