Whatever the team implementing the class put into it. If you want to see an example, grab the source code of one of the open source containers and look.
If this is to satisfy some curiosity, that's fine. But this type of knowledge is not necessary to use Servlets. That's the beauty of interfaces. As long as the implementing class does what the interface intends, the actual implementation is irrelevant.
subject: what is inside servlet.ServletRequest interface