One of the weirdest things about Russia's relations with European far right is, while "great replacement" is, in general terms, nonsense, USSR's history genuinely include some episodes that resemble the GR narratives. They were usually euphemised as "ethnicity / nationality policy", and they involved #massdeportations, especially deporting people from the Soviet-occupied countries in Europe to the east of Urals, and counterwise, mass transfers of people who may have had other, more specific, ethnicities before but who got described, and sometimes self-identified, as Russophone after the fact, in the reverse directions. American fans of "great replacement" don't seem to be very aware of them, but European near-Nazis bring these things up as "examples of the great replacement being real" with annoying regularity.

This pattern is a significant part of the reason why several areas of Ukraine were dominated by Russophone and sometimes Russian-identifying citizens of Ukraine: these areas contain large mineral deposits, so extraction and industrial facilities were built there, and Stalin & co transferred large numbers of people from other places controlled by USSR to work in these facilities.

A somewhat similar large-scale transfer was arranged by Putin for shipping large numbers of supposedly loyal Russians to Crimea after Russia occupied it.

These transfers are not the whole reason, though. Another part of the puzzle is, in times of (relative) relational calmness between the countries, there's oftentimes functionally been a linguistic and, to some degree, cultural, slope between East Ukrainian people and West Russian people. Numerous people have even had two names, one in Ukrainian, one in Russian. For just one example, the Ukrainian spacenautics expert behind the original Sputnik moment, Serhiy Korolyov (Сергій Корольов) often figured as "Sergey Korolev" (Сергей Королёв or Королев) in Russian, and because of the Russian supremacism of USSR, Soviet, documents.

OTOH, when things get ... let's say, not quite so calm, people's need to get off the slope and define themselves as one or the other tends to suddenly go sharply up, and one of the curious outcome of the last of these waves (there have been several between Ukraine and Russia over the centuries) is, there's now a large number of families who live, or used to live, near the border, where some people self-ID as Ukrainian, and some, as Russian. In other words, in the border areas, many Ukrainian people now have Russian relatives, and vice versa. Such families are rarer in West Ukraine, which, in turn, contributed to the relatively lower prevalence of people who consider themselves Russian in the West Ukraine than in the East even before the latest active phase of Russia's 2014 war on Ukraine.