This particular study (AHS-2) was initially funded by the NIH (and has continued with NIH and mixed funding): https://publichealth.llu.edu/adventist-health-studies/videos...
Your government. Health insurances. A university. Any of the thousands of billionaires on this planet.
Insurance companies don't fund medical research AFAIK? But if they did, it would be coming out of your premiums.
The US Government is not a reliable source of research or funding these days.
Universities are generally looking for subjects that will further their prestige and expand the frontiers as they understand them. This isn't that. And they, too, have funding issues, tracing in many ways right back to the government.
Fun fact, there's only about 3000 billionaires on the planet. It's a pretty exclusive club.