This is information for you as a British citizen, who would like to continue living in Sweden after 31 December 2020. There is also information here for those who are family members of such a British citizen.
As a British citizen who wants to continue living in Sweden even though the UK has left the EU, you must apply for residence status. This also applies to your family members. Half way into the application period, there are still many people who have not sent in their application.