and how does one come to know this?

> If you look at the health of the US population with a declining life expectancy, rising obesity, immunological diseases, and cancer, some self learning seems like a good thing rather than to leave your health up to institutions, companies, and doctors

Could you show your work? Some steps are clearly missing between the premise and the conclusion

Life expectancy is dropping in the US according to official studies you can easily find. It is now lower than China. Obesity, immunological diseases, and cancer have also been rising over the decades. I am not going to pin anything to specific causes as that will spur a debate too long for my attention. IMO the system is generally failing us. Without your health, you have nothing. It is too important for me to completely trust to others especially with how our system is currently incentivized.

Piggybacking off the parent, this is a major issue that medicine doesn't seem to have figured out how to deal with. Some people are so in touch with themselves they can tell they are getting sick before they have any measurable symptoms. Other people go months or years without knowing something is seriously wrong. Because modern medicine is about systematizing decisions (everything is a code), the patient's own internal sensations are given little weight compared to the measurements of an external instrument. The result is that if the particular set of tests which were ordered show nothing abormal then it can be hard or impossible to get treatment.

And many people derive imagined diagnoses from their anxieties, then come up with confident narratives based on "researching" online. Most of us are terrible at self-diagnosis, doubly at determining the causality to wherever they've arrived. There's a reason the double blind standard was a critical innovation.