Short answers, pulled from the story.
The final retreat of the Weichselian glaciation ended around 14000 years before present. This event created a large lake at the bottom of what is now the isthmus as the massive ice sheet dammed the surrounding land.
The highest point on this terrain sits atop the Lembolovo Heights moraine at approximately 205 meters above sea level. No mountains exist here, yet steep hills rise in specific places where the glacier left behind heavy deposits.
The Treaty of Nöteborg signed in 1323 established a border between them along rivers now known as the Sestra and the Volchya. This agreement defined the political boundaries during the medieval period when ancestors of Baltic Finns wandered to this region possibly around 850 CE.
The entire Karelian population of about 422 thousand people evacuated to other parts of Finland during this exodus. Finnish troops had been completely withdrawn from the area by the 26th of March following the Peace of Moscow signed on the 12th of March 1940.
The population of the Karelian Isthmus today numbers slightly less than 3.1 million inhabitants total. About 2.4 million live in St. Petersburg while a little less than 700 thousand reside in Leningrad region.