The layout source-code is usually complex,
so it’s probably better to just read the documentation.
However, if you decide to study the layout source-code,
you can easily find it by using the navigating capabilities
of your IDE.

Here is an example:

Find a Column in your code and navigate to its
source code. To do this, use command+B (macOS)
or control+B (Windows/Linux) in Android Studio or IntelliJ.
You’ll be taken to the basic.dart file.
Since Column extends Flex, navigate to the Flex
source code (also in basic.dart).

Scroll down until you find a method called
createRenderObject(). As you can see,
this method returns a RenderFlex.
This is the render-object for the Column.
Now navigate to the source-code of RenderFlex,
which takes you to the flex.dart file.

Scroll down until you find a method called
performLayout(). This is the method that does
the layout for the Column.