What exactly is meant by PPM002, PPM003, PPM004? My Macbook Pro's Apple Diagnostics currently reports PPM002, PPM003 and PPM004 error codes, which are associated with the on-board memory.
Apple's page on this topic is not illuminating.
Is there a technician's or engineering manual somewhere that documents these?
Apple doesn’t share technical documents like that outside being a certified repair tech. In the end, if the RAM is soldered to the board, it means call for a repair but I understand wanting to see high quality repair manuals, they are awesome for learning how things work and how they break.
I don’t know of any place that broke NDA and shared the manuals covering this - so you probably would get an answer indirectly by engaging  Apple support. Based on what they fix, you may be able to learn what was wrong, but most repairs do whole board swaps and don’t go into sub component repair.