ok, fair enough. You're right: Some countries have a law that "everything under the ground" belongs the govt. Britain, for example, is one of them. Anything (resources, mineral rights, etc...) belong "to the queen".
Yet detecting is common-place there, as you know. Now of course, as you also know, they don't care anything about individual coins (even gold ones! even 2000 yr. old ones! etc...). They only care about, and take (with proper recompense) hoards or something truly worthy. Is that to say that the "queen" doesn't "own" (have rights to) the invidual coins those British md'rs find? No. She/they do have rights. It's just that they don't care, and will laugh you off the steps of the museum, and tell you to keep it.
Mexico is another country which .... technically ... .also has such laws. That's why if you own land in Mexico, and OIL is discovered under your land, ...... you're not going to be rich. The govt. comes in, kicks you off your land (or takes it, or buys you out with a pittance, or perhaps "allows" you to continue living there), while they drill for the oil, and you get nothing. (I suppose that oil example would apply to Britain too, eh?). Contrast to the United States, and if you found oil on your land, presto, you're going to be a millionare
HOWEVER, even inspite of such laws, metal detectors are a common site on Mexican tourist beaches. And in-land too. Even though ... "technically" (just like Britain), all those "resources" under the ground, belong to Mexico.
Therefore, when someone cites such rules (like you're doing for Germany) you have to take into account the actual-in-practice *realistic* attitude going on. And if you don't .... and "assume" all such verbage prohibits you from metal detecting, then .... you're going to be surprised indeed, when you leave your detector at home, yet show up on your holiday vacation in Germany, only to see that others are detecting, dealers exist, clubs exist, etc... Huh? How then are they doing it? Do they not keep anything? Do they give all their stuff to the govt.?
Here's the rub though: If enough of those German md'rs go seeking clarification (because yes, technically such things *could* apply), it would only be a matter of time, before some German govt. archie is "pressed with answering this question", and will implement specific prohibitions. Thus .... sometimes ...
less visibility, scrutiny, and dumb questions are best for our hobby.