Question

what is an app which lets you build your own apps

Answer 1

https://play.google.com/store/apps/details?id=com.besome.sketch

Answer 2

You have an app called APK Editor which lets you to modify applications.

For Android, It's the only choice you have.

For PC, I would recommend Android Studio.

For bit more advanced, you can use Visual Studio.
Hope this Helped.
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