More along the lines of "Internet Explorer's <video> support will use WebM if you install a 3rd party codec because it hooks into Windows' video codec system" vs "Internet Explorer only supports h264 for <video> because it's all linked in directly to the browser." Unless you were referring to IE as the 'operating system.'
In exactly the same way that running Chrome hooks into Window's "program execution system", or installing a new video card hooks into windows "driver extension system".
Microsoft provided an API, and the WebM plugin implements it. That's how this sort of thing is supposed to work. That it doesn't any more (at least, not in the world dominated by a particularly popular mobile device vendor who shall remain nameless lest I be flamed into oblivion) is what I found depressing.
