How to create app like "Reframe"

Day 1, Day 2, Day 3…Day 30
30 Day challenge to track users completing tasks for each days.
Examples of tasks could be …1. Workout/Exercise, 2. Daily Motivation, 3. Log status, 4. Video/Audio, 5. Reading etc,.

3 streaks in a row status …how to create?

Which aspect do you find most challenging? Have you created the app yourself to try out your idea?