Where should I start to understand how computers work?
You're asking a very broad question that isn't particularly easy to answer "correctly".

What you are describing is what you learn if you follow through with a degree in computer science or maybe more precisely in electrical engineering and computer science.

So I would recommend you to do exactly that:  Go to you favorite university websites and look up what the content of a degree in EE/CS is.

For example, I've heard that MIT has a degree in EECS, so I looked it up, and found a long list of courses.

There I found a course called 6.01 —  Introduction to Electrical Engineering and Computer Science I which even has a downloadable lecture notes document:


  6.01 Course Notes, Spring 2011 (PDF - 4.9MB)


in which the basics of circuits are covered in Chapter 1.



Summa summarum you're asking for what goes into a degree in electrical engineering and computer science, and the best way to find the answer to that is to go to the source.

Universities and schools exist for a reason. :)