Path is just another environment variable, and is stored in the registry along with all other environment variables. This StackOverflow answer identifies those registry keys.
The system environment variables are stored at:
The user environment variables are stored at:
The system environment variables apply to every user on the machine. At runtime, the user path for the currently logged in user (you can find other users in
HKEY_USERS\) is added to the end of the system variable (so, the final path is equal to "%SystemPath%;%UserPath%"). Unless you specifically need every user to have the same environment variables, it is heavily recommended that you only modify the user-specific ones.
As pointed out in the comments, the path variable is special in this regard. Other user variables will override the system one.