When it comes to computers I always had something of a blind spot: I know how memory works, what Boolean logic is, how a computer adds and subtracts, I know what a bus is, what registers are, how to program in machine language, etc. etc. However, I never really quite figured out how the CPU makes data go from RAM to the ALU and, after processing, back to RAM. I always had a vague idea how it works but the control unit with fixed control paths or driven by what is called microcode pretty much remained a black box. Recently I started looking into this topic again and found a number of sources that explain in simple words how a processor works, including the control unit.
An incredible resource I found is a book called "But How Do It Know – The Basic Principles of Computers for Everyone" by J. Clark Scott. I wouldn't have thought it's possible but within 30 minutes with this book understood how a control unit in a CPU works (based on my previous understanding of how all other parts worked). And I didn't only understand only sort of how it works, but how it really works. The book describes how a CPU and memory works in less than 150 pages and although that might be considered short it goes into the details down to the gate level. And it does it in a language that can be understood by anyone even without prior knowledge of electronics. Over decades I tried to understand how this works and always had to abort my efforts at some point. And then the mystery is solved by the book in 30 minutes. It's almost shocking as is the price of only 16 Euros for the paperback version.
There's a 20 minute video on Youtube that is based on the book, also highly recommended. While the video is great, you should keep in mind, however, that the book goes into much more detail without becoming complicated or boring. Yes, I am very enthusiastic about the book, it has been a real eye opener.
While the book describes a traditional 'wired' control unit with gates, some processors also use a "microcode" based control unit. That sounds even more complicated but if you have good prior knowledge of how a CPU works (e.g. by reading the book above) and then have a look at this project that shows how to build a CPU on your own that uses a microcode based control unit you'll see that a microcode based control unit is actually simpler to understand than a traditional control unit with gates. Another revelation for me!