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.