I see no common rule across Android questions on IS SE. For example my post about possible infection of Android device was considered off-topic, however this typical broad question post which gives generic recommendations is perfectly on-topic. What is the point?
It's because your question was specific about the process for how to determine how this one app was installed.
Broad general questions of security on certain devices are on-topic here. But "how do I do X on system Y?" are better asked on the sites dedicated to that system (Server Fault for servers, Super User for general consumer devices, Ask Different for Mac, etc.)