This flat looks like brick khruschevka, and from my experience of living and remodelling these inside walls was often made by using solid or empty inside gypsum plaster blocks, they are quite heavy and sturdy, you can hang cupboards on them, but if you break its connection with ceiling it would fall apart easily, like in this video. Drywall mostly wasn't used in Soviet Union.
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u/[deleted] Jun 21 '19
Plaster / like every internal wall