Much of their analysis is based on inheritance records and obituaries on social media and in other outlets. (Their data since February 6th, however, is based on trends, as their access to official records has been disrupted.)
Die Zahlen basieren aus Erbschaften und Todesanzeigen, wobei offenbar die Daten über Erbschaften nicht mehr veröffentlicht werden.
https://archive.is/K8se7#selection-1837.276-1837.501