Short answers, pulled from the story.
Southern Russia refers to the southernmost geographic portion of European Russia, generally encompassing the Southern Federal District and the North Caucasian Federal District. These two districts contain nineteen federal constituent entities within their borders.
Eastern Slavic tribes such as the Antes inhabited Southern Russia at least from the 3rd century. The Caucasus region had been populated for millennia before these early migrations arrived in the area.
A territory named South Russia existed briefly from 1919 to 1920 during the Russian Civil War. This entity spanned the southern parts of the Russian Empire including portions of Ukraine before ending by 1920.
Six disputed regions exist within Southern Russia that are currently occupied or claimed by Russia since 2022. Donetsk People's Republic, Luhansk People's Republic, Kherson Oblast, Zaporozhye Oblast, Crimea, and Sevastopol fall under contested status.
Most of Southern Russia fits into the North Caucasus economic region officially. Exceptions include Astrakhan Oblast, Kalmykia, and Volgograd Oblast which belong to the Volga economic region.