Erm... Katherine II invited Germans to settle in Russia because it was too darn empty, noticable migration between ~1750 and 1850, other migration periods, settling in various areas, Soviet Union happened, WWII happened, Germany offered everyone considered German by the Soviets/Soviet-aligned countries, or living there and being sufficiently connected to German culture, a passport.

Even though called "Russian-German" they're from all over the place, also Kazakhstan, also, of note, Ukraine. And Russia of course. Before the war they were a unified community (all, after all, spoke Russian), then suddenly it mattered where exactly they came from. This documentary is a recent snapshot of their community, there's an earlier one but it doesn't seem to be online any more (blame German public TV laws the private channels complained, court had to give them right, and the government so far hasn't fixed the situation).