Recurrent narrative about the illegal annexation of Crimea and its integration to Russia.
Ukrainians are not foreigners in Crimea, the peninsula is an integral part of Ukraine.
The so-called referendum, that took place on March 16, 2014 was not recognised by the world. On March 27th, 2014, the UN General Assembly adopted a resolution in which it stated that the referendum in Crimea was not valid and could not serve as a basis for any change in the status of the peninsula.
See here for the EU statement on the fifth anniversary of the Crimea annexation.
Read a similar case alleging that land ownership in Crimea is an internal Russian issue.