How exactly does an isomorphic application work?

Question

React

How many "tutorials" I have not looked at - everyone rushes to write code and exclaim that everything works "magically", but this is not enough for me. I can't find a good explanation for what's going on under the hood.
I understand that on the server, the router determines where we are and renders the components in a string that it gives to the client. But it’s not clear what for then on the client to load bundle.js, which now seems to have a lot of superfluous. In general, it is not clear what is happening on the client side. Well, he got a piece of ready-made markup, what is it that optimizes the first call to render ()? And further already the client for itself?
If you have come across a sensible explanation of the entire system as a whole, please share the links!