Initial Environment

[LiNova]

I think I have a pretty good grasp of how the init process starts up, and how the bash startup files work. However there are a lot of initial environment settings which are not included in the bash /etc/profile /etc/bashrc etc. For example, the PATH is already set to a long string of directories (different from init PATH ) before etc/profile even executes. How/where are these environment variables set? Are they hard-wired in the login program, or are they user-customizable without a rebuild of login? Is this aspect of startup fully documented anywhere?