How does trip adviser (hotel or rental booking site) work?
I’m going to hazard a guess that TripAdvisor and similar websites do in fact have their core data onboard, and do not do realtime web crawling or web scraping.
In general, most such core data originates from:
In the case of TripAdvisor, a large part of their added value is crowd sourced, collective intelligence, in other words human feedback, something like Folksonomy. (Incidentally, this same kind of feedback loop was largely responsible for the initial success of Lonely Planet guidebooks, thanks to their loyal fan base.)