Russia didn't expand into Siberia in any meaningful way until after the discovery of North America. In the early yearsof expansion the true size and shape of Siberia wasn't really known...it was just miles and miles of miles and miles. It wasn't until 1725 that Peter the Great decided someone should determine for sure if Siberia and North America were actually seperate.

Vitus Bering spent 5 years sailing the dangerous coast of Kamchatka without ever seeing land to the east....but proving that the two continents were seperate.

It wasn't until a second expedition was funded that Bering eventually found Alaska in 1741.

The survivors of the second expedition (it didn't go so well, Bering lost his food, his ship, and then his life) brought back sea otter furs from Alaska and found they had a lucrative market. This opened up the Russian fur trade to the New World.