RN's not really core to Meta tbh. The most important surfaces are all built either natively or with NT / Blocks I think. Also ~19 months outdated though.
I was at Meta when this was tweeted and I think he must be referring to mostly internal apps. AFAIK the only part of the FB app that was heavily React Native was FB Marketplace.
By 2021 there were edicts in some departments that absolutely no new projects were to written with RN, with the emphasis moving towards their server-side rendering frameworks (Native Templates and Bloks.)
How does it work. In the sense that what does it provide the client side. Does the client get served pre-rendered interfaces of some sort?
Trying to figure it out as the Meta apps are very nice UX wise and maybe web technologies have come a long way to "feel native" hence i am in disbelief at the leaps and bounds being made.