Well, booking.com has an IT staff of several thousand people. But then they don't only have an app, but a site also, that makes a difference.
I buy my e-tickets for a train (if I need them) at the site of the railway company. And my wife went to the site of the airline to book the trip to her sisters house in another country.
What are your criteria for the best booking app? Like the most hotels, or the lowest price for a given hotel, or the nicest GUI or the fastest app or the best customer service in case of complaints or the option to pay in bitcoins or ...
If You want to build a booking app like railway ticket, flight tickets, hotel booking then what would be cost?
And also tell me which company provides the best booking app.