I got contacted by a recruiter who said have checked my hackarrank profile. They invited me to an onsite interview in May 2017. The onsite interview is not so technical, some questions about how you plan some daily tasks etc, they mostly presented the organization and culture of the company and I had the opportunity to talk with several teams, to be honest, the second part is far more interesting than the interview itself.
First, people emphasized many times that Booking is a business company but not a tech company, I didn't understand it in the beginning but it became clear after several rounds of talks. So as a backend developer, whatever you do need to contribute somehow to business, otherwise it's not worth doing, writing tests or refactoring for example is no good, people also don't like using TDD or CI stuff to manage the 2+ millions lines of code and 1700 IT engineers, Booking has its own fast and flexible deployment system which is to deploy on prod and rollback quickly if you noticed any error on close monitoring. Check well beforehand if this fits with your IT culture.
Second, teams are usually composed of around 8 people of different roles, UX researcher, designer, copywriter, product owner, FE, BE etc. Most product owners I met do not have a very tech background, some worked before in business consulting companies, but in Booking they need to define the priority of tasks and organize all the code production process, also almost half of the team do not understand coding. Personally I don't think this is a very enthusiastic environment for geek.
Third, I noticed that many IT employees in Booking were foreigners from developing countries (Brazil, India, Kazakhstan, Romania etc), they did not study in Neitherland before but got relocated just because of this job in Booking. And no one I met had worked there for no more than two years. I wonder if just the turnover rate is quite high there.