Each state gets a certain number of districts, depending on the size of the population. Within the states, each district is supposed to have roughly the same number of people.
Basis for districts are the counties, but since the electoral districts are supposed to have similar population sizes, counties often get merged or divided. In the aforementioned example, an electoral district was created out of the entirety of the independent city of Brandenburg an der Havel, and portions of the counties of Potsdam-Mittelmark, Havelland, as well as Teltow-Fläming.
The crucial bit is that naming practices vary between states, for though constituencies are drawn federally, they defer to the states for their names.
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