Only apps placed in /system/priv-app can install other apps in the background. The only way to add an app there is to be Google or a phone manufacturer, or be a

Only apps placed in /system/priv-app can install other apps in the background. The only way to add an app there is to be Google or a phone manufacturer, or be an end user, root your phone, and modify the filesystem (directly or through Magisk mounts). Users of unrooted phones can't install apps with the ability to manage other apps.