What is the location named outside $PLUGINSDIR in the installer

is it $EXEDIR ?

If so, $PLUGINSDIR should be located under
$EXEDIR\$PLUGINSDIR, right?

The anwer is YES.
The description of CopyFiles answered my question.
https://nsis.sourceforge.io/Reference/CopyFiles

$PluginsDir is $Temp\nsxxxx where xxxx is some random letter.

$ExeDir is where the installer is running from. Its full path is $ExePath.