“Stool transplants” are a real and effective treatment for people who are having certain types of intestinal problems. They are one technique used to help restore a healthy balance of good bacteria in the intestines. But you must realize that what you seem to believe is in the realm of urban legend and not supported by facts.
It is not urban legend. Not sure where you heard that. It is now a requirement to exclude obese stool donors and to tell stool recipients during informed consent that they may gain weight as a result of the transplant.
There was a book in the sixties they were true stories of residents at Bellevue in NYC.
They had a woman patient who was dying of intestinal issues. They harvested some healthy stool, a blender and a quart of chocolate milk and healed her.
No one ever knew. I think the book was called The Making of a Surgeon.
Google bacteria and an autoimmune disease such as multiple sclerosis or rheumatoid arthritis. You’ll be surprised at the recent studies.