The markup is used to identify what the content is. When a human sees
2003 Boxster, Red with Black convertible top, 26,000 miles, $14,000
we know someone is trying to sell something and we know what the item is, the description, and price, but search engines may or may not know this is a product for sale or is a review of a product or something else. The schema.org allows you to tell search engines this is indeed a product for sale and here are the attributes of that product including item name, price, etc.
So, in answer to your question, the markup goes in the body of the page and marks up the content.
Best,
Christopher