All the problems on the community page are created by your fellow learners on Brilliant. You can join them at any time by clicking the "start contributing" link at the top of that community page.
From there, you can create new problems, open discussions, or even write your own wiki pages! You can also add images and LaTeX formatting for formulae and equations. Additionally, you can find the general guidelines and formatting discussions for these questions, discussions, and pages at the links below.
Once your problem is published, others will be able to view and solve it. After a few folks have tried it, a level will be automatically assigned.