Hi Alberto,
Thanks for your feedback and questions.
Some of the elements can be hidden with custom CSS while others can be overridden with functions. You can find some examples here on the forum, for example:
Create custom CSS for Checkout form – [mphb_checkout]
Link to services on checkout form (or pop-up info window)
Check-in range Hours
When you override some checkout section within a function you can add code to the functions.php file of the child theme, to a third-party Snippets plugin, or create a custom plugin where you will add all your custom snippets.