They're living in Ukraine because before the creation of the Soviet Union, Ukraine was part of Russia and was known as a region, not another country or anything. Then they were in the Soviet Union, and the government would send people to different parts of the country where population was low or they wanted to develop infrastructure further, like nuclear reactors or science towns. So over the course of the Soviet union's existence, the government sent a lot of Russians to Ukraine despite the fact it is culturally district from Russia, with its own language and everything. Then the Soviet Union fell, Ukraine became a country, and a bunch of these former Russian ethnic groups were now living in Ukraine officially, and have continued to do so. They are Ukrainian, it's just that there are pockets of people there who claim Russian heritage or ethnicity.