Flattening in general is removing the nestedness in a collection. Consider you are given a list of lists. If you want to remove all child lists and get their elements into one single-level list, you need to flatten that list.
Let’s begin with :
.flatten()
This is the most simple way of flattening a list of lists into just list. Just call .flatten() on the receiver collection to have a flat list.
.flatMap()
Or, if you want to apply some kind of transformation whilst flattening, you can use .flatMap().