Noob question re: partials

Hi there, I have been searching these discussions for a while trying
to find an answer to this question. I am learning rails and have been
analyzing basecamp a bit more trying to figure out how they did
different things. One thing that I noticed was that the URL, once you
are inside of a project, always has the project and id in the url.
So, I click MY Project, and the URL goes to this:

/projects/my_project_id/project/log

now when i click the to do's tab, the URL goes to this:

projects/my_project_id/todos

What I am wondering is what controller action is being routed to here?
Is it the Projects/view action and then that action renders partials
based on what comes after the project ID? Or is it something else?

Its kind of late an I hope this makes sense.

Thanks!