Russia and South Korea are Capitalist, extremely so. Did you think the USSR never dissolved and did you mean to say North Korea?

The PRC is a Socialist Market Economy. The model is described as a birdcage, the CPC allows markets to naturally develop but only along their guidelines, and increases ownership as competition creates these new monopolist syndicates. Socialism Developed China, Not Capitalism is a good article going over China’s economic model. The CPC has the power it has as a Dictatorship of the Proletariat, it needs that power to maintain supremacy over their bourgeoisie. Communism is achieved by degree, not decree.

The "no true Communism/Socialism" bit is from "left" anticommunists, generally, who haven't read theory. As for Imperialism, the PRC and Russia don't meet Lenin's definition of Imperialism, hence why I linked the book in the first place.