remove website input box from comment section and modify header

Hi there,

try adding this PHP Snippet to your site:

add_action( 'after_setup_theme', 'tu_add_comment_url_filter' );
function tu_add_comment_url_filter() {
    add_filter( 'comment_form_default_fields', 'tu_disable_comment_url', 20 );
}

function tu_disable_comment_url($fields) {
    unset($fields['url']);
    return $fields;
}

in which page do I've to add it? and how do i modify the comment header?

That code is PHP.
This doc explains how to add the PHP:

https://docs.generatepress.com/article/adding-php/