I ask this because I know from a previous post on askhistorians that there are various theories as to why feudalism didn't take such a strong route in sub-Saharan Africa as it did in European countries and Asian countries but one of them was The size of Africa made it so that there was enough land for farmers to move to if the landlord or local chief was too cruel. Why didn't the same thing happened in Russia considering it's size?
I can give you a partial answer specifically to your second question.
Serfs weren't actually allowed to move around by law. While there hadn't been uniformity in law around these issues earlier, in the Sudebnik of 1497 (a set of laws introduced by Ivan III responsible for the move towards centralization of the Russian state and establishment of a more uniform feudal system), freedom of movement for all serfs was restricted to the week before and after Yuri's Day (in which serfs could switch estates provided certain obligations were satisfied), also known as George's Day in Autumn, a holiday in Russian Orthodoxy based on the end of the harvest.
It also established a break-away fee (pozhiloye), which allowed serfs liberation from their feudal lord after a set of payments (which most obviously couldn't afford).
More than 50 years later the Sudebnik of 1550 increased pozhiloye and introduced taxes associated with punishing peasants for refusing to submit the harvest to their masters. Tsar Ivan IV introduced forbidden years in 1581 as a result of regime difficulties associated with both the economy and with military forays. This temporarily forbid serfs from moving between estates, and ended up permanent as it was never repealed. With the Ulozhenie of 1649 freedom of movement was fully and completely banned. This rule remained until the mid 1800's.