How to get specific keys from mongoDB collection?

Question

MongoDB

Let's say a collection in mongoDB looks like this:

directory: directory: {
  subdirectory: {
    SubSubdirectory: {
      ...
    }
    ...
  }
  someString: 'lorem...'
  ...
},
anotherDirectory: {
  ...
}

The nesting levels can be, say, up to 20.
The sizes of strings (someString) can be from a few words to, say, 100 thousand characters.
How can I access only the keys of this entire collection? And not to all at once, but by levels: first I need only two root keys (directory, anotherDirectory, ...), then nested keys in the directory (subDirectory, someString, ...)
and so on
How to do this?