How to add custom rewrite rules and point to specific templates

You need to create query_var for sector and you need to create a rewrite rule with RegEX. I created a plugin for this. Create a file and add into the wp-content/plugins folder and activate it.

<?php

/*
Plugin Name: Custom Rewrite
Plugin URI: https://serkanalgur.com.tr
Description: Custom Rewrite
Version: 1
Author: Serkan Algur
Author URI: https://www.serkanalgur.com.tr
License: GPLv2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
*/

class FilterRewriteRule
{
    public function init()
    {
        $this->InitRewriteRules();
        add_filter('query_vars', [$this,'add_query_for_sector']);
        add_filter('template_include', array($this, 'filter_template'), 99);
    }

    private function add_query_for_sector($vars)
    {
        $vars[] = 'sector';
        return $vars;
    }

    private function InitRewriteRules()
    {
        add_rewrite_rule(
            'products/import/([^/]*)/?$',
            'index.php?pagename=products/import&sector=$matches[1]',
            'top'
        );

        add_rewrite_rule(
            'products/import/page/([0-9]{1,})/([^/]*)/?$',
            'index.php?paged=$matches[1]&pagename=products/import&sector=$matches[2]',
            'top'
        );
    }

    public function filter_template($template)
    {
        global $wp;
        if (get_query_var('sector') && is_page('import')) {
            $custom_template = locate_template('sector.php');
            if ($custom_template) {
                return $custom_template;
            }
        }
        return $template;
    }
}

register_activation_hook(__FILE__, function () {
    $writing_ideas = new FilterRewriteRule();
    $writing_ideas->init();
    flush_rewrite_rules();
});

add_rewrite_rule add the rewrite rule with pagination support. filter_template locate your sector.php file and redirect the request to this file. query_vars adds sector query to WordPress system.