Uhm, USSR was very much socialist. It officially strived to reach communism, but the actual economic system was socialist. It’s often called communist due to the ideology claiming communism to be the perfect and inevitable endgame for the country, which is a big disambiguation. Suffice it to say, communism was never reached.

China, on the other hand, is capitalist, despite being ruled by a communist party. It has a private property on means of production, which defines a capitalist country as opposed to socialist or communist one (source: Wikipedia)