Well Frederick the Great was literally Frederick the Great of Prussia, so yes it is prussia which was the most powerful german state though there were a few other powerful ones including bohemia. Further adding to the case that "germans" are really Prussians are the facts that Prussia and it's politicians were primarily responsible for the eventual unification of the german states into modern germany namely Otto Von Bismarck in the early 1870s. In reality no Germanic state government was responsible for major colonization of the americas or anywhere else on the scale of G Britain, Spain, France or the Netherlands even, but German peoples settled independently without their government, in religious groups, or as retired mercenaries granted land (Hessians fighting for brittish in the War of USA independence for example. So I think of the Germans civ as a hybrid of cultural german settlers of the new world, and a powerful prussian state that decided in a twist of history to colonize.
No the Germans are not the Holy Roman Empire, and there was no basis for the Holy Roman Empire being included in the game. They did even less colonization (which is the main topic of this game as we all know) and they were before the time period(they did exist until 1800 but not in a big way, and not as a part of modern Germany really). Also they encompassed a lot of Europe at their height not just Germany. The Military portrayed in the game as "Germans" is a pretty close representation of prussian era military although I am sure they had Musks and Grens in reality as well at least for a time. So it's definitely modeled after Prussia and represents Prussia more closely.
BTW i hereby submit my application to be Chief Historian of ESOC
https://en.wikipedia.org/wiki/Prussiahttps://en.wikipedia.org/wiki/Unification_of_Germanyhttps://en.wikipedia.org/wiki/Otto_von_BismarckGermany has been through a lot of changes politically though even in the last century with it's territory changing in both great wars.
ORiginaly people thought about incorporating German speaking Austria into the Empire, but Austria was never interested. Not that any of germany was ever interested, it was unified by conquest military and political.
An interesting side note illustrates how convoluted the political history of the German peoples is from the Knights templar until today:
Franz Joseph Hayden was a little known court musician who lived in rich estates composing songs for their owners mostly. In those days without recorded music composers did not usually become much renowned in their lifetime. He wrote a piece for the birthday of Francis II of Austria for his birthday"Gott erhalte Franz den Kaiser", and Francis II loved it so much he made it in effect the national anthem of the newly formed Austrian Empire over which he ruled. The austrians as I noted have always been resistant to being unified into north German states. And yet today, that same hymn which was originally one of Austria has had words added to it and is the National anthem of modern day germany "Deutschlandlied"
https://www.youtube.com/watch?v=YJfFtGPoACI(it says anthem of the holy roman empires, but thats because for a few more years he ruled the HRE until it was dissolved and he came to rule just Austria )