I think the answers to your questions could vary greatly depending on the baby but I will share how it applies/applied to Evan.
Evan had an upper endoscopy (EDG- looking @ esophagus + stomach + part of small intestine) & also a colonoscopy (self explanatory-lol). He had these done @ the same time.
Ped GI suggested he have this when he was around 8wks old b/c of the continued blood, mucous, green stools… despite 4+ wks of strict TED. He initially just wanted the lower (colonoscopy) but then he & general ped decided to add the upper as well to rule out esophogael thrush.
They can show all sorts of things… for us they were mainly looking for: irritation in the esophagus & inflammation colon. They took biopsies as well.
It’s minimally invasive but they do have to put to sleep w/ light anesthesia. I posted a while ago about our experience w/ great detail if you’re interested… here’s the post.
https://www.infantreflux.org/forum/forum_posts.asp?TID=11394&PN=1
Scroll down toward the bottom where I update.
HTH