has fixed buildings (no camping in this case) and a variety of offers for activities outdoors and indoors,
targeting mainly groups of children and youth, but also adults can use the place,
accomodation mostly in segregated arrangements of (small) houses (at least in this case)?
It definitely is not a youth hostel. In the linked thread it is discussed that “summer camp” does not fit. tourism=holiday_resort seems also wrong (“Holiday resort” seems to be more like something like “Center Parks”).
It is argued that calling it “holiday village” is too much an incorrect literal translation from the Germen “Feriendorf”, so that’s why asking here, how this type of facility would be called by native English speakers.
A summer camp is a kind of program that could theoretically operate out of one of these facilities, but if it operates year-round, then it would just be a camp. That’s a very overloaded word, especially in OSM, but it’s a situation where (at least in American English) we don’t have a more specific word in common use. I’m not sure if it applies here, because it implies a sort of rustic character away from modernity.
Alternatively, if the facility is devoted to more contemplative or relaxing activities, as opposed to something engaging and exciting, then it would be a retreat center or retreat house. (This is not to be confused with retreat=*, which is a misnomer for refuges.)
It can be. For example, this YMCA camp, this Girl Scout camp, and this Boy Scout camp all put visitors up in cabins, no camper vans or tents allowed. Two of them are tagged leisure=summer_camp because they mainly operate during the summer, but I’ve considered tagging the YMCA camp as the same to avoid a plethora of tags to cancel out tourism=camp_site like impromptu=notent=nocaravan=no.
To me, “resort” implies luxury, which may or may not fit here. I think the main question to ask is the purpose of sending children here: does it serve a vaguely educational focus (e.g., outdoor education, morale building, intrapersonal growth) or is it primarily for leisure and entertainment?
There certainly are plenty of luxurious resorts, but there are also rustic and/or budget oriented resorts (for example: https://www.rusticcottages.com/). To me the word “resort” implies vacation, recreation, or other sort of retreat from day to day life. I agree that a somewhat educational focus would be a different sort of place.
Both, groups use it in different ways. But I would say definitely more leaning to the first.
It is more “luxury” than “cabins”, it is more like dormitories here. And: Catering is provided by the facility. And there are also communal rooms, seminar rooms, multipurpose sports hall, wood workshop, photography lab, etc (besides the outdoor sports facilities). Is this still a camp in English?
The YMCA camp I mentioned earlier has all these things too, other than the woodworking shop and photo lab. However, that camp’s marketing emphasizes the educational activities more than any particular amenity. I suppose a facility could be versatile enough to accommodate the full range of programs from retreats to summer camps to family leisure recreation, in which case we’d probably just need to pick one category and clarify through other tags.
For me, it would rather be a leisure=summer_camp. Apart from the level of luxury (which a resort typically delivers), the key difference is the intended audience: a resort is open to any paying customer, but summer_camps (unfortunate “summer” attribute notwithstanding) only cater to organized groups.
As in many other fields, OSM taxonomy for tourist facilities grew up organically (cf. different keys for tourism=hotel and leisure=resort although they’re similar things), often with Anglocentric focus, so you often have to shoehorn atypical cases (such as this one) into one of existing popular tags. Of course, “any tags you like” always holds, but personally I prefer using an existing tag, however imperfect, rather than inventing new ones.
OpenStreetMap entity, 
