how importing words and phrases wrapped with gettext?

Show me the code where you have defined this sentence as a translatable string.

<?php
/*
Plugin Name: MySite-plugin
Author: Sacconi
Description: Site specific code changes for example.com
Text Domain: mysite-plugin
*/
/* Start Adding Functions Below this Line */
function wpb_author_info_box( $content ) {
 
global $post;
 
// Detect if it is a single post with a post author
if ( is_single() && isset( $post->post_author ) ) {


......


......

$titolo_prezzo_include="<h4 class="titolo_affitti">". esc_html__('Il prezzo include:', 'my-text-domain') . '</h4>';

my-text-domain is not your text domain. You defined your text domain as mysite-plugin. So your code should be changed to:

esc_html__('Il prezzo include:', 'mysite-plugin')

This is a fundamental of WordPress i18n. Coding help is outside remit of my support here, but if you have any problems using Loco Translate’s string extractor, then feel free to reopen this topic.

As a side note, WordPress source strings are conventionally in English.