sorry for asking a few stupid questions...i alway heard pple mentioned that develop phone apps to sort of earn of passive income once the app is sold to the buyer.
Searched online but it pop up a few links which i click here and there then it somehow need to pay to attend the app development course?


1. So I must attend an paid course to learn app development?
2. Once sold the app, the passive income last forever or for 10 years only?
3. Need to go which platform to sell the app?
4. How is the sold app's passive income being calculated?