most of them are defined in relevant files in the /etc directory. running grep -r ^[[:upper:]] /etc/* must list most of them.
a few particularly interesting files according to the Ubuntu documentation are:

1. /etc/environment: documentation states

"...This file is specifically meant for system-wide environment variable
settings. It is not a script file, but rather consists of assignment
expressions, one per line..."

2. files in the /etc/profile.d directory ending in .sh. documentation states:

"...An alternative is to create a file for the purpose in the
/etc/profile.d directory... Files with the .sh extension in the
/etc/profile.d directory get executed whenever a bash login shell is
entered (e.g. when logging in from the console or over ssh), as well
as by the DisplayManager when the desktop session loads..."