Understanding the React Source Code Pt. 1 is the first of a series currently up to five parts at the time I'm writing this.
I'm not going to lie to you, I flipped through the second part of this and it was kind of heavy sledding. You are probably only interested in this if you've used React for quite a while already and had considered trying to go through the source yourself to try and understand it better.
In that case, this will simplify your task. However, if you're really looking for a higher level overview of how the internals of React work, this is probably not what you're after. In fact, if you find something like that, let me know about it and I'll link to it here. But if you want something that digs right in, you should check this out.