I don't really understand the politics of Eastern Europe. The Nazis annexed countries with the excuse of Lebenstraum but I don't really get why the USSR didn't officially say countries under their control belonged to them
The Nazis and the Soviets has different ideologies and therefore behaved differently. The Nazis supported the expansion of Germany into a greater Germany in which other nations land and resources would become used for the Aryan race, (Germans).
The Soviets were "anti-imperialist" and so did not believe in annexing whole countries into the USSR even if they exported the "socialist" economic system to them. Eastern European countries became part of the Warsaw Pact (a military alliance) and Comecon (an economic alliance).
There are so notable exceptions such as the incorporation of Latvia, Estonia and Lithuania in to the USSR at the end of World War II. What is now "Belarus" was the part of eastern Poland that the Soviets invaded in 1939 was also incorporated into the USSR.
There was a lot of problems with post World War II Poland as there were large forced relocations of millions of people as Poland was "compensated" for the loss of its eastern territory to the USSR with formerly German land in the West. The Germans living there were simply "evicted" and forced to move into East Germany so ethnic groups and territory lined up in accordance with Soviet Communist theory on nationalities. I have read that Stalin wanted to incorporate all of Poland into the USSR and was defeated due to polish opposition/resistance but I can't find a source for that.
