You need to use pre_get_posts hook. Where u can include the default category posts on other category pages. 

    function wpse_pre_get_posts( $q )
    {
        // its the main query, and its the category page archive
        if( $q->is_main_query() && $q->is_category() ) :
    
            // get the default selected category id, fallback to 1 (default one id)
            $default_cat = (int) get_option('default_category', 1);
    
            // if the category page is not the default category page
            if( $default_cat != $q->get_queried_object_id() ):

                // get the already used tax_query arguments
                $tax_query = $q->get('tax_query');
    
                // add the default one
                $tax_query[] = array(
                    'taxonomy' => 'category', 
                    'field'    => 'term_id', 
                    'terms'    => $default_cat 
                );
    
                // relation should be or, which means either the current category or the default
                $tax_query['relation'] = 'OR';

                // set the values to query
                $q->set('tax_query', $tax_query );

            endif;
    
        endif;
    }
    add_action( 'pre_get_posts', 'wpse_pre_get_posts' );

That's it. You are all done