How to avoid silent app installations I have gone through this post Silent app installation wherein it clearly mentions it's highly impossible to self install the application without going through the whole installer proceedure.But when i read this one
Remotely Installing Unwanted Apps on Devices
i didn't understand how this is even possible

I am aware of the android applications run in a sand box environment so one application cannot install/interfere another until and unless it has an process Id or something/referencing and also should have the user permissions to do so but with the recent silent app installation  by some OEM is making me doubt about this security issue.

When one app can then why can't other apps follow suit .

Please avoid answers like don't use Internet at all/root the phone/stop purchasing the devices of that particular make ,company.
The above link might help in some clear understanding
The only way to be sure is using an firewall or manually read every permission for every app installed on your phone and use an permission revoker app both requires root.

You will search for any app that have the permission "download files without notification" and either revoke that permission (it might cause some bugs) or either just block them out of the internet using an firewall like AFWall+