Used Googletranslate to understand. Sorry for writing in English.
That´s not correct in Total.
You are right you can not just directly use any sub from another App.
But if the app supports a/some Intent(s) which can be used then it is Possible.
Remember this (old but still valid) Tutorial
DISCLAIMER: This tutorial is written without testing code. There may be bugs, but try to understand the process, not copy the code. An intent is a way for different applications to communicate at run time. It can be used to pass messages between activities and make them do actions. Using...
www.b4x.com
The point is that you need to develop both apps to fully Support some communication. Or at least you need to know the possible Intents the calling (or the receiving) App understands.