In client-side application whenever we require to use any external application we use HTTP Requests.
Allows different software components to communicate with each other. It enable the front-end of a web application to send requests to and receive responses from the back-end.
There are various Language options with which we can implement the backend, hence there must be a simple mode of Communication ie, Message Passing Language/Notation.
When we open BMS, then the app gets details from another API called MOVIE INFO API.
After selecting the seats, and we checkout, the BMS uses an BANK API, where it generates bill and take the payment.
So the BMS is using these APIs and utilizing them to provide a service i.e, Movie Booking.