There's tons of folk remedies that do absolutely nothing useful at all, too. When you don't have any reliable medicine, you take whatever you have on hand and hope for a placebo effect. Eventually, you find something helpful because even a broken clock is right twice a day.

There are recorded beliefs in medieval Germany, for instance, that carrying or wearing an eye from a bat will make you invisible.

Folk medicine and remedies are one thing; traditional and herbal medicine certainly has its place and is understudied.

Homeopathy however is pure nonsense even on a fundamental scientific level.

It is unfortunate that the two get conflated.

What’s the physical basis for that?

Homeopathy is not a "folk" tradition, it is simply an insane concept.

Can you give an example of a well-known homeopathic and/or folk remedy that has been adopted into regular medicine, maybe in the last 20-50 years?

Obligatory Mitchell and Webb https://youtube.com/watch?v=HMGIbOGu8q0