They use cheap, poor quality parts. In addition, many things will be proprietary designs, so it makes upgrading nearly impossible. You won't be able to find what they put it it because it's proprietary. You might find out who makes it, but that won't really tell you much.
I would definitely just build it yourself. By picking your own parts, you can ensure quality components and can customize it to your exact needs. Plus, by putting it all together yourself, you have a much better understanding of how it works, allowing you to easily customize the settings to enhance performance.