How can an existing user login to check on previous orders? I see where an existing customer can login during the checkout process for a new order.  But where can they login to check on existing orders?  Or is that even possible?

I tested and it appears that the Drupal site logon is not the same as the Commerce customer login.
The Drupal site login and Commerce login are definitely the same; check the login details again.

Commerce 2 supports checking the details of orders out of the box.

It adds an "Orders" tab to the user profile (/user/UID/orders), and a view (that you can customize if you like) is provided to display the order history (/admin/structure/views/view/commerce_user_orders/edit/order_page).