Does factory reset remove apps that install into the /system from an unroot phone Suppose the following situation:

End-user have a unroot phone. (the phone is not rooted).

End-user install a app, which actually is a System app and is installed in the /system of the phone.

So, does a factory reset remove these app?

(In the first place, can end-user install a system app onto a unroot phone?)
System apps can't be installed without root.  Only updates to existing system apps can be installed, and they don't actually reside in the /system partition; the original app is kept there, which allows you to uninstall updates separately.  So a factory reset will remove any updates, but not the original app.