I don't know how to find code of widgets provided by Microsoft in Azure DevOps so I can edit, merge and play with them.
Already checked the documentation and google.
I don't think they are open-source. I couldn't find them on github, which is where they would be if they were. That being said, there are quite a few marketplace extension that I do know are hosted on Github. Find one that has support page linking to github that interests you.
If you haven't already, look through this documentation for adding a dashboard widget.