Up until now, Apple has only displayed customer reviews and other points of interest in Apple Maps from Yelp. However, the company seems to have expanded its sources to include reviews from TripAdvisor and Booking.com as well.
In the United States, Yelp reviews still continue to dominate in hotel listings with reviews from TripAdvisor and Booking.com supplementing it. But in other regions of the world like London, Paris and Sydney, Booking.com reviews are being primarily displayed to users for majority of the hotels listed on Apple Maps.
Apple launched Maps back in 2012 to disastrous public reviews. Since then, the company has worked steadily on improving its mapping service by improving its coverage and adding 3D imagery for key places around the world. Based on Apple minivans that have been spotted quite a few times in the United States, it is likely that the company is working on bringing Google Street View like functionality to Apple Maps in the near future as well.