How does ajax request actually work?

Question

when we type url in browser , the browser follows the rules defined by Http and construct the Http message in an appropriate format like header and body , this is how request leaves the application and travels through the other network points.

In case of Ajax , how browser makes an asynchronous request? or is it directly handled by the Javascript engine embeded in browser, and  he only construct and send the http message to the network without any help of other component of Browser , internally??