Understanding how a web-based API (HTTP POST) functions. Android and iOS client apps.

An android and iOS client app communicates to a server database via HTTP POST requests to an API and receives responses. Documentation for these functions and how they work is limited. There are about 20 functions. I want to know how each of these functions work both independently and co-dependently in terms of carrying out the action initiated by a particular aspect of the android or iOS app. I want to know the arguments (including how they are generated), type, and rules. This will involve documenting action/function/data request and response/dependencies/data flow including use of diagrams.