De facto or de jure?
If de jure, than neither USA nor PRC are authoritarian because both have multiple political parties.
If de facto, than both USA and PRC or authoritarian. USA in particular is governed by an unelected establishment of NatSec/Finance/Oil elites; an oligarchy in the purest sense. Who elected Victoria Nuland and Jake Sullivan to orchestrate the Ukraine wedge?
Authoritarianism is government by absolute (or near absolute) control of a single body. North Korea, China, Russia being prime examples.