Short answers, pulled from the story.
Northwest Russia is roughly coterminous with the Northwestern Federal District, which administers the area. It is bounded by Norway, Finland, the Arctic Ocean, the Ural Mountains, and the east-flowing part of the Volga.
By 10,600 years before the present, the Fennoscandian Ice Sheet had left Russia entirely. The ice reached its Last Glacial Maximum extent in northwest Russia around 17,000 years before the present, five thousand years later than in Denmark, Germany, and western Poland.
The route ran from the site of Saint Petersburg east up the Neva River to Lake Ladoga, south along the Volkhov past Staraya Ladoga to Novgorod, across Lake Ilmen, and south via the Lovat River and portages to the Dnieper and onward to Kiev and the Black Sea. It formed the main axis of Kievan Rus'.
The western side of Northwest Russia was the main source of squirrel, for which there was a large demand in the Middle Ages. Luxury fur, especially sable, came mostly from the northeast of the region.
The original population spoke Uralic languages. Baltic-Finnic tribes were collectively known in Russian chronicles as the Chudes. Distinct groups included the Ves' east of Lake Ladoga, the Vychegda Permians, the Voguls, the Ostyaks (Yugra), and the Samoyeds of the far northeast.
Zavolochye means "beyond the portage." It was a geographic term referring to the area between Lake Onega and the lower Dvina River, named from the perspective of travelers who had to cross portages to reach it.