I have been interested in APIs in the last time and i want to know exactly how it works from the server side.

so i tried to figure out how the client side API working ,
first step i took the Google Plus One JS client side code and i tried to figure out how it works , mostly i tried to find how it gets connect to the server side.
i couldn't find what this whole code doing , mainly because i focused to find Ajax requests , but there is none of them.

this is the api I checked :

https://apis.google.com/js/plusone.js

now , my questions is simple ,
i want to know how API connect to the server side , do they use AJAX?
what exactly they use to send request to server side ?


i need you're help to know more on how api working on Client side , mainly on the connect to the server.
so , how it get's done?