What is the best source of guidance on when and how to conduct an FCA and a PCA... and what prerequisites, resources and documentation are required?
There are two primary sources on the purpose and conduct of Functional and Physical Configuration Audits (FCA/PCA).
The new MIL-HDBK-61B CONFIGURATION MANAGEMENT GUIDANCE, 7 April 2020, has updated guidance on the purpose of the FCA and PCA.
The IEEE Standard for Technical Reviews and Audits on Defense Programs, IEEE Std 15288.2™-2014, has detailed direction on the requirements, criteria, and application/practices for Defense reviews and audits, including the FCA/PCA.
If you have (or create) an account in the ASSIST Database https://assist.dla.mil/ , you can download a free copy of the IEEE standard. There's a video on how to download this on DAU Media at https://media.dau.edu/media/0_vz1a5v8f