Anything written in python would be available in a python node. The out of the box nodes and any other package that uses zero touch nodes, where there is no nested graph to investigate, are likely written in something else, like C#.
As you can see from @Draxl_Andreas’s post, most of the more well known packages have a GitHub repo that allows you to see all their work, including an official Dynamo repo.
For helpful information on python you can check out this thread: