What do you mean by how does this site work? Do you mean how does the forum technically work?
If that is the case then I suggest you start here: https://www.namepros.com/help/
If you mean just how do you get the most in terms of information from the site, then the answer is a little more complex and depends on where you are in your domaining experience so far. Definitely start in the beginners section: https://www.namepros.com/forums/domain-beginners.25/
After that take time to explore other parts of the forum. Look at discussions about domain sales. Also look at valuations. Definitely feel free to ask around for help, but bear in mind that most of the newcomer questions have all been answered many times, so definitely use the forum's search feature.
Summary: You can buy, sell and learn all about domains in this forum. You can also partner with or hire designers and developers. Each section has it's own rules which you'll usually find at the top of each page.
Excellent info about the wide-world of domain related topics can be found in the above posts or, by using the NP search option for topics specific to your own goals and aspirations.