Some halachic questions

What customary law is there regarding health?!

The question is whether this is about health. After all, some claim that it is not health in the normal sense (spiritual dangers, etc.). If it were normal health, then since there is no health concern, there is indeed no reason to keep it (see the column on legumes).

What is the explanation for the fact that medicine has entered the Halacha, if the thing is dangerous it is forbidden because it is dangerous, and why else should it be forbidden?
(Regarding meat and fish, as I believe the Maga already writes in the G. that the natures have changed)

This is not accurate. See Here:
https://mikyab.net/%D7%9B%D7%AA%D7%91%D7%99%D7%9D/%D7%9E%D7%90%D7%9E%D7%A8%D7%99%D7%9D/%D7%94%D7%9C%D7%9B%D7%94-%D7%95%D7%9E%D7%A6%D7%99%D7%90%D7%95%D7%AA-%D7%9E%D7%94%D7%99-%D7%9E%D7%95%D7%9E%D7%97%D7%99%D7%95%D7%AA-%D7%94%D7%9C%D7%9B%D7%AA%D7%99%D7%AA/
The question is What level of risk is prohibited? This is a halachic determination.

“Not health”, does the meaning mean that it does not cause leprosy but something else (meaning that something else is something else), or does the meaning mean that leprosy is actually a spiritual thing?
If option b’, then all our ability to rule on halakha is limited because perhaps there are actually hidden explanations that are not related to what is written.

I didn't mean to ask what this has to do with Halacha, but the prohibition is not supposed to be specific but includes all risks at level X, and from now on you should ask a doctor whether such a thing is a risk at level X or less.
Therefore, there is no prohibition of meat and fish, but a prohibition of danger, and then you are supposed to check objectively whether it is dangerous or not.

After all, many of the fearful claim that even though it is not forbidden, the nullification of the reason does not nullify the regulation, and so I ask why suddenly the Sages would establish a prohibition of meat in fish, at most they are supposed to say that meat in fish falls into category X and is therefore forbidden, and whatever turned out not to be true/the natures have changed, what is the point of leaving the prohibition that was essentially not a prohibition “in the hafza” on meat and fish, but on danger, and this is not a danger.

Yishai,
The hidden flavors method is well-known and I don't agree with it. But that's when it comes to a law whose taste is clear. There, the hypothesis that there might be a hidden flavor is not reasonable and there is no reason to fear it. But in mixing meat and fish, there is no clear flavor. The sages talk about danger and it is not clear whether this is a normal health hazard or some kind of spiritual hazard. Therefore, here it is not necessarily related to the hidden flavors method in general. Maybe it is and maybe not, which is why I also wrote just to be content with that.

Itay,
But it is impossible to determine a sharp X level that is halakhically prohibited, and it is also very difficult to determine the level of medical risk. And has anyone determined what the level of risk is that would permit eating? And is there a doctor who will tell you what the level of risk is for this disease and this one if you don't eat? There is no way to do this, neither on the halakhic side nor on the medical side. Therefore, the sages sometimes establish a prohibition on a certain act that they believe falls within the scope of the prohibited risk. Therefore, it is possible to see meat and fish as a prohibition and not just as a danger. Although, as mentioned, it is certainly reasonable to see this only as a danger, and then it is not mandatory if it turns out that there is no danger.