Drupal reuses its hash salt
I have migrated other CMS's to Drupal and had experience moving user accounts.  Drupal's password algorithm is extremely good.

The salt you are referring to in the settings.php is not used for passwords. The drupal_hash_salt salt is used for generation of things like cookies.  Hence why it says you must keep it the same across all web servers if you are in a load balanced setup.

Generation of passwords uses the SHA512 algorithm with randomly generated salts using openssl php libraries (not mt_rand or other weak random number generators).

If you want to learn more about password generation you can start here reading the API documentation for Drupal

You see that in the function to generate it calls a "private" function to generate a new password salt (_password_generate_salt()). Which in turn calls a lot of other private, custom Drupal functions (it is a long rabit hole).

So in general the password scheme used for Drupal is extremely deep and nested, it does not use PHP functions considered to be "weak".