Is the implementation of XMLHttpRequest different per browser?

Please, help me to figure out this, I was curious about the source code of methods like XMLHttpRequest.open() as per

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest,

after some (relatively) lazy googling I wasn't able to find out how methods of that API are implemented, I found references to some code on c++, for example here nsXMLHttpRequest. My question is, are the implementation of that object especific to browsers? A beginner question, certainly. Thanks if you can help me to understand how this all stuff is related.