How does Virtual DOM work in React, and how has it been organized before?

Question

JavaScript

How was spa organized before? After all, with the advent of React, etc., everything has become much easier (as it seems to me). Before React, there was also a global, local state, etc. What is in React? And did everyone do it in their own way? And now it turns out the angular react, etc. have become like the Official Documentation on how to build web applications. So that everyone does not implement in their own way, right? Regarding the work of the Virtual DOM (inside, at the molecular level), can there be articles to read ?! Thanks