Thu Ubuntu linux distribution is developed in the open.
You might already know that not only the source code of the linux kernel is open-source (and of course developed by a whole lot of other people unrelated to ubuntu), but the design process of the userland tools, of many libraries, services, and desktop environment is also somewhat open.
Go to the development platform launchpad.net (you might need to register to get access to more features), and then at Ubuntu Blueprints you can see a sortable table. Click on column head "Delivery" (= Completion status). For the completed items, often a lot of design documents, requirements, review docs, bug reports, etc are listed or linked.
Maybe pick an item that is easy to understand, e.g docs concerning implementation of the "Ubuntu One music store", (now gone).
For newer items, less is available.