The simple answer is that the 11 CPTPP members — Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam — will decide whether to allow China to join the pact.
The next step is for the current members of the CPTPP to accept China's application make a review, then recommendation to current members wether to accept China into the club. Take note that member countries of CPTPP are concurrent members of the recently passed/activated largesr trade alliance in the world, RCEP.