Has your dll files been damaged, but the site still works Then that would be due to the caching of IIS and/or your browser.
Or are you curious to how a site could work with only .aspx and .cs files The answer to that is basically that: when a request is made by the client, the .cs is compiled and cached in memory.
(note that there is more to it than that. It's just a very much simplified explanation)
