Okay, let's back up and restate. In this walkthrough, we've told you four main things.
One: How to get the MySQL source.
Two: What's in each directory in the source.
Three: The main sequence, as one walks through the server code.
Four: What physical structures look like.
We worked hard to make a description of the MySQL source that is simple, without distorting. If you were able to follow all that we've said, then that's wonderful, congratulations. If you ended up thinking that MySQL is really simple, well that is not what we wanted to convey, but we think you'll be disabused of that notion when you have a look at the code yourself.