Where is the "add to cart" configuration?
Go to the 'Order item types' page and click the "Manage form display" link for your order item type. You will then see a tab for the "Add to cart" form display.

For the 'default' order item type the url will be: /admin/commerce/config/order-item-types/default/edit/form-display/add_to_cart

There is an open issue for finding a more discoverable way to expose these settings.