If you now look at your Shop page and open a detailed view of any product, you will discover that there is still no cart functionality in it. The reason for it is that the Products module does not know that the Cart module has been installed and is ready for use. Let's connect two modules.
1. Open the admin area of the Products module (Content | Product Manager).
2. Click on the Preferences tab.
3. Select the Cart module from the field Cart Module.
4. Click on Submit and open detailed view of any product. You should see the button Add to My Cart in the detailed view of the product. If the product has some attributes additionally to the quantity field, then the attributes are displayed in a drop-down field besides it.