Catherine the Great was a German princess who ousted the original Tzar in a coup. She also happened to be the ruler who vastly extended Russian power and began the Russian occupation of Ukraine, Crimea and Poland.
For all intents and purposes, she may as well been a Russian the same way Napoleon’s identity is irrevocably tied to France even though he was ethnically an Italian.
It’s the same with Lenin and Stalin. Trotsky, less so because he was Jewish.
Lenin was Russian. Trotsky was Ukranian. Stalin was Georgian. That they represented different nationalities was essential to Soviet ideology, which saw itself as global-multicultural and opposed to the Empire, rather than nationalistic. While Catherine reclaimed Crimea from the Turks, Crimea was Russian long before Catherine the Great; calling the Russian Empire’s rule of Crimea an occupation is like referring to the British occupation of Manchester, or the American occupation of Pennsylvania. The Rus were in Crimea before they were in Moscow!