You seem awfully interested in everything going on inside your device which is fantastic!
Grab yourself a copy of ifile, or filza and go to town exploring your iOS device.
You have root access, you can literally see every single file.
Go look in the what's app containers and see if there are images there as well as in the dcim folder.
Get diskpie and let it give you a nice graphical display of where your space is being used… usually the bigger pieces of pie are where media is stored…
Also, honestly, go back about a year or two on this forum, from when it was still iOS 5 and read some of the threads. There was a lot more information being shared and posted back then and you might discover/learn quite a bit. A lot of it still applies even on the newer ios's.
Also go to r/jailbreak over at reddit. It's a constant stream of activity with thousands of daily users. A lot more exposure to your questions. Direct contact with developers. A lot of really "smart" people
I am not really interested (was back in the day when I was closer to the "tech" & "code") these days (too much stuff in life to handle), but have been forced to... Too much inflow of Media on whatsapp is filling up my 64GB iPhone.. which I thought would be impossible unless I loaded things I want (which I havent done yet).
I have iFunBox and stuff to go into the file system. I can go looking for files. I cant dissect how and what logic whatsapp uses to "download" "redownload" "save locally" "save in camera roll" "not save" .. things it does upon "delete / trash in whatsapp conversation / group" vs "delete in camera roll".. Sometimes it redownloads media.. wierdly
I've played with it in various ways, but the "LOGIC" (code which I can reverse engineer) is not clear/ apparent to me.
Also, under windows I could attach a "file/ disk system" monitor to a program and see what its doing on files/ file system/ disk. With the iPhone, I need to find some "devs / hackers" and/ or their forums where I can tap into some of their wisdom around this.
PS: I will look for the older threads you mention, but do point them if you can please as I can always end up looking at stuff, which is no where what you intended for me to look.