This goes over environment variables and variables in general:
If you're looking to have a env variable set for you when you login to a shell this is done in either the .bashrc or .bash_profile file located in your home directory.
The kernel stores the list of environment variables and their values for each process, and inherit them to child processes. They exist at runtime, and are not stored in some file or so. But there is a virtual file in
Which contains all the environment variables. The kernel makes them visible by that virtual file. One can list them. For example to view the variables of process 3940, one can do
cat /proc/3940/environ | tr '\0' '\n'
Each variable is delimited by a binary zero from the next one. tr replaces the zero into a newline.