More dry reading - search for TSO C77b

https://drs.faa.gov/search?modalOpened=true

It's the FAA regulation that describes the data required to certify an APU. I'm still looking for released configuration description documents that will give more systems details....

Pages 18 - 23 here: https://vdocuments.net/b777-quick-reference.html?page=19

Bleed air system impacts: https://vdocuments.net/b777-air-systems.html (pages 10 and 23)

General systems: https://vdocuments.net/b777-systems-and-expanded-qrh.html (Page 47)

http://www.b737.org.uk/apu.htm

here's a description for a 737. all the same principles will apply here. Fair warning....it's a bit dry.