How to store system app's settings on /system partition? I have a system app, and I need that both the application itself and its settings shall be in /system partition.

I can change the application to be in system by moving from /data/app to /system/app.

Yet, I see that the configuration/settings are still saved somewhere in /data (not sure where).

Is it possible to change it to be in /system? If yes, how?

Thanks.
You can't. Configurations / settings / cache / databases of every app (system or user) are stored on /data or in /sdcard/Android/ for some apps. That's why /system partition is of limited size as compared to /data. Android works that way. Apps aren't built to read and write settings to /system partition as it holds a read-only filesystem, only changed during OTA updates. A read-write /system partition won't fit into Verified Boot  mechanism implemented by Android for security and to prevent malware attacks.

Further reading: Android Partitions and Filesystems