Question: To create buttons in the SDL Tridion ribbon toolbar, where can we find the extension related information (like the config file, js files) of already existing controls like Check-in, Check-out, Undo Check-out so we can use them as a reference?

Answer: 
Parts of the configuration are in the `..\Tridion\web\WebUI\Editors\CME\Configuration\CME.config`, the rest is in the `..\Tridion\web\WebUI\WebRoot\Configuration\System.config` I believe.

The ASPX, CSS and JS files are all located in `..\Tridion\web\WebUI\Editors\CME`, interesting paths to follow there are the `Views` and the `Views\Popups` but also the `Controls` might contain useful items for you to investigate.
I don't have a running system to check with right now, but I believe you can find this information by "following the trail" from System.Config. Just like with our own extensions, the Tridion CME must register its commands in there, and will have links to js, css, config, etc.