> why it does things the way it does is relevant -

Yes. React automatically detects the minimum set of changes to a DOM that is required to update it's state, and manages how those updates are applied to maximize performance. It transparently handles both app state changes, and basic changes to views triggered by updates to models.

This is a problem everyone in webdev has. Everyone.

Did you saw any of the comments here on how a little JavaScript allows people to update the DOM?

> the reason facebook had problems with state management and modularity was the sheer number of engineers working in a single shared codebase (I was there, it was miserable)

No.

The reason why Facebook had problems with state management and modularity was because Facebook had to maintain websites.

The reason why Facebook developed React was because they had clever people who identified the problem and were given resources to work on a way to turn a hard problem they experienced into a trivially solvable problem.

Companies, big and small, develop their own tooling and infrastructure to improve efficiency, and they do it all the time.