This request is not mundane, but peeps might be able to help.
Where do I find a map of Europe from 1900 and find out how the boundaries changed over the following fifty years. I would like to know if Prussia was a country or an Empire, and the name of that area now.
I am the first to admit that my knowledge of European (geographical) History is very limited, possibly non-existent.
Most of Germany was a collection of city-states and small principalities / duchies until united by the Kaiser in the 1800-ish. But with the Kaiser being Prussian, they sort of took precedence in the 2nd Reich. The majority of the senior German military officers in WW1 were Prussian, as the region had a very strong military tradition.
The eastern borders of Prussia yo-yoed around a bit, mainly in lockstep with the western borders of Poland and, at one point, the Baltic States. That part of Poland is called Pomorże, and includes Gdynia and Gdańsk. In the inter-war years, Gdańsk (Danzig) was a Free City with neutral status - accorded to it in the Treaty of Versailles.
Prussia came into being thanks to something of an ooops moment by one of the Polish kings in the middle ages, who invited Teutonic Knights to help fight against the Tartars and gave them land to settle as a thank you. The kingdom, as it was, came to an end with the formation of the Weimar Republic in 1919.