What type of an algorithm is being used for directory structure scanning?
For most file systems, a directory lists its entries in a certain order, so reading ("scanning") an individual directory will return its entries in that order.

But when it comes to scanning whole directory structures, there is no such thing as "core scanning": there isn't a common implementation for that.

I think depth-first is more popular; e.g. GNU find only supports depth-first scanning.  This may be due to depth-first scanning consuming less memory on average; it can't be due to breath-first search being harder to implement, as it's just a matter of treating your list of files to visit as a queue rather than a stack.