That is a good question. Here are some hints on how to get started adding code to Studio:
1. Use cms/urls.py to find the view that handles the page you are interested in
For example, the URL for the main course outline page is /course/COURSE_ID. You can see the URL definition here:
url(r'^course/{}?$'.format(settings.COURSE_KEY_PATTERN), 'course_handler', name='course_handler'),
2. Find the view handler
In this case, the method is called course_handler, which my IDE tells me is defined in cms/djangoapps/contentstore/views/course.py. If you look at the "GET" clause, this in turn calls the function course_index.
3. Determine the template being used
Looking in course_index, you can see that it renders the template "course_outline.html". You can find this file in cms/templates/course_outline.html
Is that enough to get you started? When you are ready to start writing code, there's a lot of useful guidance here:
I hope this helps.
- Andy