You can learn more about how this feature was built by reading Rapid's PR#1221
Map Roulette is a map editor similar to Rapid. In map Roulette users can solve Challenges which are composed of Tasks to be carried out. Once a user finishes all the tasks in a given challenge, the challenge is considered to be complete. You can learn more about it from their documentation.
The Map Roulette UI works by showing users Task Clusters in a given area. The tasks in a cluster can belong to one or more different challenges. Users can click on any cluster to zoom into it and break it further into smaller sub-clusters.
Users zoom in like this until they reach a point where there are no more sub-clusters, and the map only shows individual tasks. Here's an animation of how that works: