How to create apps and sell them, I did it for ChatyBoty, Family AI iEight, ww3 game and for Live Traffic Map and Weather 2019.
First of all you need to download android studio for android app development and Xcode or Appcode for iOS development.
Then you have to search online for current implemented ideas similar to your idea and just do more from what they did, make it better and promote it with google, facebook and other social media apps.
You don't need an advance knowledge for java and swift, you have to just start and the knowledge you are looking for most of the times is free over the internet. We are living in 2019 with millions of android and iOS app development tutorials.
In addition, If you have many downloads you can sell an app for 10000$-20000$ and sometimes even more than 20000$.
Hope you find my article useful.
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