As of December 16th, the mobile version of Brilliant.org does not easily show topics. The desktop version shows all of the topics you currently follow on the left hand side. These can include Algebra or Mechanics. The mobile version does not have an easy way to direct you to these individual links.
Sometimes I want to solve only Algebra problems. It is very hard to navigate to this page, and so far, the easiest way is to copy a link from the computer to a mobile device and bookmark it.
Hopefully there can be a tab added in the side window alongside Home, Started Problems, Favorites, etc.