How to add .html to the end of a link?

Question

Apache HTTP Server

There is a site URL
1st https://my_site.com/remains as it is.
2nd level: https://my_site.com/my_second_page.html
3rd level and above:

https://my_site.com/my_second_page/my_third_page.html

Tried:

if ( !strpos($wp_rewrite->get_page_permastruct(), '.html')){
    $wp_rewrite->page_structure = $wp_rewrite->page_structure . '.html';
 }
// некоторые ссылки тогда битые

All links are deeper than the 2nd level, add .html at the end of the url, how?
Most likely you need some kind of rule in htaccess. I have already tried everything in WP, including hooks, although I could be wrong.
This code only changes the URL of the product, but the category does not work for some reason:

// для продукта
function wpse_178112_permastruct_html( $post_type, $args ) {
    if ( $post_type === 'product' )
        add_permastruct( $post_type, "{$args->rewrite['slug']}/%$post_type%.html", $args->rewrite );
}
add_action( 'registered_post_type', 'wpse_178112_permastruct_html', 10, 2 );
// для категории, когда идем на страницу категории там ОШИБКА 404
function wpse_178112_category_permastruct_html( $taxonomy, $object_type, $args ) {
    if ( $taxonomy === 'product_cat' )
        add_permastruct( $taxonomy, "{$args['rewrite']['slug']}/%$taxonomy%.html", $args['rewrite'] );
}
add_action( 'registered_taxonomy', 'wpse_178112_category_permastruct_html', 10, 3 );

Answer 1

If I understand the question correctly, then you need to put something like this in the settings (Settings->Permalinks->Custom Structure): https://site_name/%postname%.html