Well, that's pretty deep lol, but I can simplify and just answer your general questions
Full understanding of the cycle of parasites and pathogens must first be understood.. Ie, why and when and how a bird would contract but not be able to fight off a pathogen or parasite without a fully functional immune system and properly functioning gut is key to that; just like in humans, our gut us one if our biggest immune responses
Ph ofca chickens first stomach can be about 3.5-4.
Gizzard ph isn't really relevant, it just grinds up the food, the digestive juices are already there, so ph really pretty much remains the same... However, if they eat something big or say, bony.. The ph can plummet down to as low as .5
Ph of vinegar depends on what kind ya get... Ph ranges from 4-6. ACV us usually smack in the middle at 5.
ALOT like people, proper PH if the gut and body overall, helps keep pathogens and parasites from being able to survive... Much like the PH and microbe content if the soil can affect whether or not the soil is infected with parasites as well. PH affects horomone response, histamine release, as well as keeping a proper balance if healthy microbes in the digestive tract.
The reason for APPLE CIDER vinegar us just because it is closer to "natural", not chemical
Hope it helped