require tubalmartin/cssmin - lib location

I have a doubt with regards to require tubalmartin/cssmin.

Reference::
Notes (8.x-4.2+)
require tubalmartin/cssmin: ^4.1

When using composer the dependency didnt get installed. So when i manually do a composer require on tubalmartin/cssmin, it goes to vendor. While in many discussion it is mentioned we must add the lib in the contrib module "lib" folder

Install advagg using composer

Thank you very much.