The "scripts" of default Nodes is lost when compiling the engine. To access the source code, you can download it from the Git (remember to change the branch when new releases come out).
However, they are not necessarily written in the same way as you would in GDscript, it is not the same language, and they abstract things away so that you do not have to deal with them. To see examples of code that use these nodes, try opening a template (second tab on project selection), or follow a tutorial.
When trying to find documentation, you can use the "Search Help" button on the top right of the script view, or ctrl+click on the thing you want more information about.