This is a recurring pro-Kremlin disinformation narrative about the illegal annexation of Crimea.
Crimea did not "return" to Russia but was illegally annexed by Moscow. On 27 March 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. On 17 December 2018, the UN General Assembly confirmed its non-recognition of the illegal annexation of Crimea.
According to international law, Crimea is a part of Ukraine. Russia violated international law as well as key principles of the European security framework by illegally annexing the Autonomous Republic of Crimea and the City of Sevastopol in 2014.