I don't know that SpaceX uses CDR/PDR etc and adheres to waterfall methodology.



Yes.SpaceX for its commercial only design efforts don't do the exact PDR/CDR events as would be a part of a government project like Commercial Crew (even though it was supposed to be commercial it still had a PDR/CDR events although very toned down from that of a program like SLS). It is interesting to note that the SLS program managers have adopted a more commercial method of detaching systems from each other for the PDR/CDR points so that the long lead build times hardware can proceed into production earlier to maintain the earlier launch date for EM-1.But the concepts of what PDR and CDR represent in the design flow is the PDR is the identification of the requirements that must be meet, and a possible preliminary design that would satisfy the set of requirements. The CDR is the point where the engineering peers have reviewed the designs and the designs are then ready to release to manufacturing. The order of which systems are to be released to manufacturing and in what order is more a dependency of the designs. Designs that are interdependent are released together, designs that are dependent on another design can be released after. The PDR and CDR are labels for the maturity levels for the designs. But they have become because of the formalness of the government usage of these as a single event in time for all the systems/subsystems of a end item, has caused a tremendous time delay in the development of very complex systems which also adds a tremendous costs. SpaceX to aviod such schedule slowdowns and added costs don't do a combined all systems PDR/CDR point but individule events that from the outside is not even observable since it is internal only and can only be discerned in the case of a CDR event that the hardware has started being produced.So the tanks have definitely passed the conceptional CDR point. Most like so has all the rest of the structure. A question would whether the engines have as well but most note that in rocketry everything follows from the engine capabilities and specs. Such that the engines have passed the CDR point some time ago. Could be just months in a fast paced company like SpaceX though.