If Facebook were built today which tech stack would it use?

This is not original question on the Internet, but all the answers there are not really helpful. I'm interested in other people opinions. So, what do you think, which tech stack is most appropriate for building such feature rich application that serves more than billion users?

Some of the things to consider are that backend languge should be fast and efficient. Which data storage paradigm and/or specific tech would you recommend? Cache technologies and techniques?

Also, there are a lot of real time capabilities such as chat, notifications, feed updates, comments (now you can even see when somebody is writing reply, before he/she even posted). Should real time stuff work via web sockets or classic AJAX with polling? Can web sockets scale to hundreds of millions scale without major downsides?

For the frontend stuff, should it be "one website fits all" regarding fitting content for all screen sizes, or to have separate versions for various screen sizes? What about UI, should it use some library or framework like Vue, React or Angular? Or maybe something else? What about rendering, should it be rendered on client or server rendered first?

What about image processing, video processing, live streaming?

You don't have to comment on everything, but explanation why would you pick something would be appreciated. I really want to see what people think and maybe focus on learning some of the stuff from the answers.