Drupal 9 requires modules to be installed using the Composer application.

Composer is installed in the same folder as the website on he host server.

Composer not only installs module and libraries, it will update them when available.

Using the host server file manager, you will findthe file Jason which has all the modules added to the website are listed in the format:

     "drupal/<module name>": "<version>",

To add a module, enter the module name into the Jason file and save it.

Then "php composer update" is entered in a ssh terminal, usually a host utility.

As a good policy, run a database Update.

Then go to Extend and enable the new module.