Find a product you are interested in and press the "Buy product" button.
All sales goes through Amazon and on their product page there is information about the seller (most products are sold directly by Amazon or shipped by Amazon) which means shorter delivery time.
All contact regarding returns should be through Amazon or the product seller.
Read more on the following page: https://www.amazon.de/gp/help/customer/display.html/ref=hp_gt_rr_riyo_en/?nodeId=201824330
Contact the seller on Amazon and ask for a replacement product or refund, if you want to make a return see "How do I return an item?".
Yes on the "Your order" page when you log in to the account created when ordering on Amazon.
In the first instance you can contact the transport company (eg PostNord), in other cases we refer to the respective seller on Amazon.