First, Google doesn’t rank websites, they rank individual web pages.
The second point is that Google does not rank web pages based on web design, they never have and likely never will.
Third, Google doesn’t rank web pages based on the amount of content, they never have and likely never will.
Google ranks web pages based on relevance and usefulness.
What’s going on is that the web pages are relevant and useful for Google’s users. Nothing more, nothing less.
The website has been around for 15 years so it is a well-established business with many earned citations, the website provides exactly what Google’s users expect to find when searching for the service offered by that website. There is no mystery here, except why you don’t see what Google and all the user’s of Google search is seeing in the web pages of this website.