The challenge selection scene is where you pick the difficulty of play. Just touch your choices on the left and right sides then go back to the main scene to play with those challenge selections.
Workday & Shifts
On the left side, select the experience level. These categories affect the number of shifts (6, 8, or 10) and the minimum required path lengths. The experience level doesn't necessarily indicate expertise at making paths. It's really more about balancing available tools, explosives, and mods to maximize your ability to make long paths in as short a time as possible.
On the right side, the selection affects the minimum required path lengths and determines the number and types of obstacles, the frequency of one-way streets, and the number of bridges and tunnels.