Android - How are "ShareIt","Xender" etc able to install APK without requiring "unknown source" permission?

I did a bit of digging and it is possible without System permissions from API 21 onwards https://developer.android.com/reference/android/content/pm/PackageInstaller.html.

In fact, there is a code sample by Google which shows how to do it. https://github.com/googlesamples/android-testdpc/blob/master/app/src/main/java/com/afwsamples/testdpc/cosu/CosuUtils.java

If the apps are installed silently (i.e.without the usual installation dialog), presumably these apps directly call pm, i.e. PackageManager, which as a system app bears the permission android.permission.INSTALL_PACKAGES.

The phone has to be rooted so the app could call pm in a root context, else you will be greeted with the message "Neither user xxxx nor current process has android.permission.INSTALL_PACKAGES".

EDIT: As @Izzy pointed out in the comment, android.permission.INSTALL_PACKAGES is also granted to apps installed to /system. See below comment and this SO question for more insight.