Can an android antivirus detect or even prevent data-leakage of a mobile phone? Moreover if the user has installed .apk outside playstore. I searched on google, but answers like this: https://www.kaspersky.com/resource-center/threats/data-theft does not answer my question...
Antivirus software on Android don't have access to TLS encrypted traffic. And there is also no way to modify the system as app or user to make TLS decryption possible.
Therefore an antivirus software can not detect any data leakage to a remote server if the app uses TLS encrypted communication.
If the app had been installed from Google Play store or via side loading does not matter.