Most people call those Deserted Island or Nook Mile Islands. They are randomly generated, and usually have either a DIY Recipe, Fossil, and/or Furniture in a tree. And may have restricted spawns, making rare bugs/fish more common.
They are fun to visit. Just be careful in overspending your Nook Miles, because you need them to buy some cool things.
They are fun to visit. Just be careful in overspending your Nook Miles, because you need them to buy some cool things.