Spylock
Spylock used a shouder-surfing proof pin-entry method to help users’ lock apps on their smartphones. I learned Kotlin and the entire Android development suits, incuding billing. I also learned a lot of UX/UI design via this project.
Spylock might still be on the play store at https://play.google.com/store/apps/details?id=com.grizai.android.spylock.
I killed SpyLock after 5 months (including learning and development) because the bulk of the user traction came from teenagers which do not have the desire to pay for this kind of services. I was not interested in advertising-based funding model.