I'm not sure what aspect you are referring to with a "long" site.
I've seen some that rely on the css "flexbox" that allows boxes to resize and shift and reflow based on the browser window sizes. I've seen some that include multiple iframe elements to pull in other web pages on the client side, or composition of everything on the server side, or a bunch of scripting to allow for infinite scrolling by loading in new html fragments.
Personally I dislike the style. I use plugins that disable scripts and cross-site loading by default, which has a positive side effect of not loading ads and other annoyances. Most "long" sites rely on javascript that invariably pull their scripts from sites outside their control, a few of those like googleapi I mark as trusted, others not so much.