On other sites, when you click a link for example, the page goes blank for a second and you see a loading indicator in your browser tab. However, on sites like YouTube and Twitch, when you click a link, you are sent to the page without the loading indicator with no white screen at all.
How is this done? I know it's related to Ajax, but what would be a modern approach to this?
It's called SPA - single-page application. The content of the page is rewritten without reloading. The content is being fetched in the background using ajax and then applied to the page with a bit of help from a frontend framework.
If you want to know more just google for single-page application.
You can change the URL in the address bar without causing a page reload using the History.pushState API.
history.pushState(state, title, url);
