The supply piping is inspected wherever it’s visible, usually in the crawl space for me. Sometimes it’s in the attic.
Vent pipes are in the attic, and in the roof, as well as watching for slow draining or hearing gurgling sounds, or smelling sewer gas.
Pipe is usually only insulated where it is prone to freezing, usually in a crawl or in the attic, but I’ve seen it insulated in the basement (over drop ceilings) to prevent condensation drips.
Bret because of the type questions I recommend taking more training on Plumbing.
That being said Supply during inspection usually starts at the meter area or supply entry (usually )front of the home if public.
Water heater is just hot water supply and cold water to unit supply which may be a branch line.
Inspecting vents depends on which components you are observing and view available as often it is in the walls till roof exit. .
(you need to learn more )
Question (3) I am not exactly sure what the question is.