Design specifications are here. All screens are in scope, both desktop (top priority) and mobile layouts. For underlying code logic and APIs you can check Topcoder App repo (contains code for the current production version of design submission page, which is close to the target design, however there are some differences), and TC Site repo (contains current production version of development submission page). For you reference you will also find in the challenge forum links to the specs of Topcoder API v2 and v3. If you see that some of the existing functionality in this pages is done using API v2, but the same can be done using API v3, please replace by API v3 way.
As usually, do not hesitate to rise any questions in the challenge forum.