Capitalism is the root of evil

The PRC is more classically Marxist than under the Gang of Four, when they abandoned materialist analysis and attempted to implement Communism through fiat. Large firms and key industries of the PRC are firmly in the public sector, while small firms, cooperatives, and sole proprietorships make up most of the private sector.

Marx didn't think you could abolish private property by making it illegal, but by developing out of it. Socialism and Communism, for Marx, were about analyzing and harnessing the natural laws of economics moving towards centralization, so as to democratize it and produce in the interests of all. This wasn't about decentralization, but centralization.

Markets themselves are not Capitalism, just like public ownership itself is not Socialist. The US is not Socialist just because it has a post-office, just like the PRC is not Capitalist just because it has some degree of private ownership. Rather, Marx believed you can't just make private property illegal, but must develop out of it, as markets create large firms, and large firms work best with central planning:

The proletariat will use its political supremacy to wrest, by degrees, all capital from the bourgeoisie, to centralise all instruments of production in the hands of the State, i. e., of the proletariat organised as the ruling class; and to increase the total of productive forces as rapidly as possible.

I want you to look at the bolded word. Why did Marx say by degree? Did he think on day 1, businesses named A-C are nationalized, day 2 businesses D-E, etc etc? No. Marx believed that it is through nationalizing of the large firms that would be done immediately, and gradually as the small firms develop, they too can be folded into the public sector. The path to eliminated Private Property isn't to make it illegal, but to develop out of it.

The essential condition for the existence, and for the sway of the bourgeois class, is the formation and augmentation of capital;[43] the condition for capital is wage-labour. Wage-labour rests exclusively on competition between the labourers. The advance of industry, whose involuntary promoter is the bourgeoisie, replaces the isolation of the labourers, due to competition, by their revolutionary combination, due to association. The development of Modern Industry, therefore, cuts from under its feet the very foundation on which the bourgeoisie produces and appropriates products. What the bourgeoisie, therefore, produces, above all, are its own grave-diggers. Its fall and the victory of the proletariat are equally inevitable.

This is why, in the previous paragraph, Marx described public seizure in degrees, but raising the level of the productive forces as rapidly as possible.

China does have Billionaires, but these billionaires do not control key industries, nor vast megacorps. The number of billionaires is actually shrinking in the last few years. Instead, large firms and key industries are publicly owned, and small firms are privately owned. This is Marxism.

As for the USSR, its economy worked quite well for most of its existence. I recommend reading Do Publicly Owned, Planned Economies Work? by Stephen Gowens, who goes over what went right and what went wrong in the Soviet Economy, including why it was dissolved. Further, GDP growth was positive throughout the near entirety of its existence, collapsing when it liberalized:

I recommend doing more research on Marixsm and the economies of the PRC and former USSR.