I'm truly sorry if it's not specific enough but I need to code an app X that is installed on my own phone and "logs/gathers" my own incoming messages from one source only (eg sms, or WhatAapp) and displays them in that app X. Information can be stored on the local device.

How to I go about this? Just some general steps or concepts for me to look into would be highly appreciated since I am not sure where to start.

It's not supposed to be a spy app or anything like that. I have some very basic understanding of JS and it's libraries but I welcome all solutions.