Facebook use React to generate webpages. It isn't 'written' in html in the way most people here would understand that; the servers don't send html to your browser, it's all javascript. Your browser generates the screen elements directly from the javascript.

It isn't 'obfuscated' in the usual sense of that word, that is deliberately obscured. It's highly compressed to reduce network traffic - names are converted to single letters and the javascript is 'minified' to remove spaces and comments. The effect is much the same!

It's the very devil to manipulate using userscripts and userstyles. For one thing the code is rarely the same (IDs, classes) from one moment to the next. Same with Twitter.

Most if not all really big websites work this way. And I assume the same React code is used to build their apps, which is why websites and apps often look identical.