Programmatically speaking, how is Android set to be "unrooted" by default? I am not asking what the process is to undergo doing it, but how it is programmed in the first place to prevent certain actions, such as:

1.Downloading to SD card from Google Play store.

2.Accessing/modifying files mounted on the /system directory, and such directories like that.

3.Modifying text, GUI components, etc.

REALIZE that I am NOT asking for code, samples, etc., I just want to know the general idea of how this would be implemented in the source, i.e., hierarchial system, privilege blocking code on the GUI end, or possibly code running in a kernel process, etc.
This isn't a special extraterrestrial thing. 


The default user doesn't have root privilege.
All those binaries are removed which can grant root access to default user.
/system partition (which contains sensitive files) is mounted as read-only. As only root user can remount a partition with R/W access, you're out of luck modifying those files..