Why Russians have been invading countries since 1930s?
The period in which Russia employed acts of invasion on other countries is not limited only to after the 1930s. Most of the reasons for these invasions shifted, however, there is one line of reasoning consistent throughout all of Russia's history, which is poor geography. Since Russia's coastlines are mostly covered in ice during winter this proves to be challenging for the trade of goods and that would impact its economy. Furthermore, all of the neighboring European countries except Finland, have vast portions of land suitable for growing crops, gas, and oil. This is especially true for Ukraine since they have the largest amount of arable land and vast gas reserves in Donetsk, Kharkiv, and Lviv.

One more possible reason is the rise of communist ideology in Russia, more specifically, the Stalinist branch, which is a revolutionary philosophy whose followers want to spread and enforce it in every aspect of a person's life without allowing them any kind of freedom. After the second world war, The United States and the USSR would have a cold war in which the conflict would not be carried between each other, but proxy states instead.  In the modern day, people claim this reason would still bear some form of relevance since Vladimir Putin, the current president of Russia was a former KGB member and would have likely never fully accepted the democratic and capitalistic system brought after the collapse of the USSR.
After World War 2 Stalin wanted to take advantage of the Army that he had built up in order to stop and eventually beat Hitler. Interestingly, a lot of support was provided to Russian army by US that enabled ths buildup.
 
Stalin used this to strong army to  take over East Germany, Poland and other East European countries.
Under the Soviet Union, Stalin did indeed take over many countries including those you mentioned. Here is some more info:

- 1934: Invasion of Xinjiang (China), which led to heavy casualties on both sides
- 1939 (1 Sept to 6 Oct): Invasion of Poland, a brutal invasion coordinated by the Soviet Union and Germany which led to significant loss of life particularly for Poland (66,000 killed, 133,700 wounded, and ~675,000 captured). For more info, see https://en.wikipedia.org/wiki/Invasion_of_Poland 
- 1939-1940: The Soviet union invaded Finland and as a result of this and other actions was kicked out of the League of Nations. In the end, they got ~11% of Finnish territory
- 1939-1945: During World War II, the Soviet Union additionally completely or partially occupied Romania, Bulgaria, Hungary, Yugoslavia, Czechoslovakia, Austria, Denmark, and Norway

Although Soviets and Germany cooperated for some time, this fell through and eventually Germany attempted and failed to invade the Soviet Union. 

Stalin was ultimately found dead on March 1, 1953, and the Soviet Union was dissolved in 1991.

Sources:
- https://en.wikipedia.org/wiki/Foreign_interventions_by_the_Soviet_Union
- https://en.wikipedia.org/wiki/History_of_the_Soviet_Union_(1927%E2%80%931953)