This discussion also takes us back to the idea of shells, which was brought up briefly in Tipsheet Vol. The child knows a lot about its parent's environment, but not everything, and once the child gets going, the parent is not able to affect the child's. The parent process knows nothing about the child's environment. This may sound more like an afternoon soap opera than a computer system, but there are reasons for it. It does tend to keep things under control, though.
Some examples are probably now in order.
We'll stick with talking about the ksh since that is the default for all Bama users. In Tipsheet Vol. In the ksh we used two files, .profile and .kshrc, to get this done. This is the file that gets read when you login for the first time in any given session. This will become the parent process for any other processes you spawn.