How to use the unzip_file() function successfully?

Apparently I could not do this in the particular area I was working in…without CHEATING!

require_once(ABSPATH .'/wp-admin/includes/file.php'); //the cheat

WP_Filesystem();

unzip_file( $zip, $dest  );

From where are you calling the unzip_file() function? External file unrelated to WP?

No, its in a script that is called through a theme activation hook.

Try this

if (move_uploaded_file($source, $zip))
{
    WP_Filesystem();
    unzip_file($zip, $dest);
}

https://php.net/manual/en/function.move-uploaded-file.php

WP_Filesystem(); remains undefined. Its a strange thing really.

I see, this article might be helpful

https://aquagraphite.com/2012/11/using-wp_filesystem-to-generate-dynamic-css/

comparing with your codes.