Again, that&#x27;s entirely a matter of application design. The general answer is that the server uses the request URI and HTTP headers to determine which fragments are needed, but there are as many specific answers as there are applications following this paradigm. (Said paradigm, incidentally, is just that of the single-page application; this is only YouTube&#x27;s specific implementation of it. It sounds like you might get more use out of a general treatment of how SPA&#x27;s work, than of a deeper dive into the details of the specific implementation under discussion; it might be worth your while to take a look at <a href="http:&#x2F;&#x2F;singlepageappbook.com" rel="nofollow">http:&#x2F;&#x2F;singlepageappbook.com</a>, which, while I haven&#x27;t actually read it in any close detail, looks to be a pretty good overview and high-level discussion of the topic.)