Extend is where Modules are enabled, updated and uninstalled

After a Module in physically installed into the site using Composer, then it needs to have its box checked on the List page.  After clicking on Install at the bottom, when the screen returns and see if there are any problems.  If not, good policy to click on Update on the Administration Toolbar and then Clear Caches.

Usually go back to the newly installed Module to see if there are configurations to make.

To uninstall a Module, click on its box on the Uninstall page and then click on Uninstall at the bottom. 

After the Module is uninstalled from the website, then use Compose to remove <module>.