What does the following aggregate query perform?

Fetches the posts with likes between 100 and 200 and sets their _id as null

Groups the posts by number of likes (101, 102, 103.) by adding 1 every time

Calculates the number of posts with likes between 100 and 200

Fetches the posts with likes between 100 and 200, sets the _id of the first document as null and then increments it 1 every time