Afaik almost a monopoly: there is Deutsche Telekom which does the same thing and Vodafone. I think apart from some local providers almost everybody else is just a reseller of one of the two.

There are resellers that do not just rebrand a whitebox product, but have their own IP addresses, network and peering polices. Their customers are not necessarily impacted by the IP peering policies of the company that owns the access network.