What would you call these? If I'm not wrong, they don't exist in the US/UK

Midwest US I would call it a “niche.” But that usage isn’t especially common. Mostly, we just say the person was buried “above ground” in a mausoleum. I had a relative interred that way and everyone just referred to where the casket was put in the mausoleum as his “final resting place” or as his ”spot.” Above ground burials aren’t especially common here, so a lot of people are unfamiliar with the terminology. Mostly I think I‘ve heard people just say that he’s “buried above ground.”

It's true most Americans have been buried underground and marked with a headstone, with cremation becoming common more recently. But many cemeteries across the country include a few above-ground mausoleums, usually owned by well-off families. Above-ground tombs are strongly associated with New Orleans, where they are predominant owing to the low, flood-prone terrain. New Orleans cemeteries are a significant tourist attraction and photographic subject. The type of structure in the photograph would specifically be called a *wall tomb*. The compartments in it could be called *niches*, *alcoves*, or *crypt*. *Niche* comes from the Latin word for *nest* (nidus) and originally referred to a recess in a wall that might be suitable for making a nest. Later it came to have the figurative meaning of any place or situation of comfort, then a specialized interest or role. But it never lost its first meaning, so I can still talk about a "niche in a wall" and be understood.

In an ecosystem, plants and animals have their own niches. A spot they occupy in the ecosystem. It can apply to hobbies, like you might have niche hobby, aka not many people have that hobby so it's like a spot you occupy among lots of hobbies.

[This is a columbarium, containing thousands of niches, and a small number of crypts ](https://en.wikipedia.org/wiki/Chapel_of_the_Chimes_%28Oakland,_California%29)

Mausoleum. I think a niche js more for cremated remais. But we certainly have what is pictured.

In it's other sense, a niche isn't a hobby. It's a specific sector, like how these are small, enclosed parts of a larger structure. A 'niche interest' is one that is not a large part of the public consciousness: sport is a general interest; in the US, handball or rugby league could be a niche interest, as could a particular genre of books or a rare craft practice like historical costume-making.

For the hole itself, niche actually works fine. For the actual structure, mausoleum is probably the best-fitting commonly understood term. Or possibly wall crypt because it’s outdoors.

Niche does mean that, too. Like a nook or a cubby, but with a known purpose. A niche is a small space specifically meant for something to fit inside.

We have lots of these "wall coffins" in the United States, they're generally called crypts and in a building called a mausoleum  https://www.memorialplanning.com/resources/burials/types-of-mausoleums/

There are different words depending on the exact configuration— mausoleum, vault, crypt, etc In my experience (Melbourne, Australia) they are most common for Italian immigrants. I remember when I was younger an old Italian friend of the family died. Lacking a better word for it, my mum told me she was being “filed away” rather than buried.

They’re called a “niche” so very similar. The whole wall is called a “columbarium.”