Within the KM Editor, under the Help menu is Open Preferences folder (which opens ~/Library/Application Support/Keyboard Maestro). In that folder is the file Keyboard Maestro Variables.plist.
As usual, make sure Keyboard Maestro and Keyboard Maestro Engine are not running on the target Mac.
Also, there isn’t a guarantee that the files will be up to date on the source Mac if you don’t quit Keyboard Maestro and Keyboard Maestro Engine there as well before copying the file. Some files will, some wont, it depends on the files.
I suspect the plist has been superseded by the database -- I don't have the former (newbie!) but do have the latter. A suspicion that is reinforced by this, from 2019.
Easy enough for you to check, though -- Get Info on both and check the last modification dates... If the plist hasn't changed in yonks, why keep it?
Please give an example of a "default, built-in variable." I've never seen one. Are you talking about tokens? They aren't variables, and aren't stored anywhere. They are evaluated at run-time.
Correct - the .plist is old and unused in modern Keyboard Maestro. Keyboard Maestro never deleted it because sometimes people revert to older versions (that's what trial periods are for after all), and variable files are generally very small.