Hi
The default contact form is build-in the theme, in content-contactform.php, but you’d better don’t modify it, otherwise, you can’t upgrade the theme smoothly in the future.
I suggest you can build your own contact form with “contact form 7” plugin if you want to add more various attributes. After you created contact form with it, create a new contact page without select “contact” template, you can customize your own contact page with visual composer and put the form shortcode into the page content.
Thanks