Firstly, we have to know that there are two kinds of flatMap. It depends on the transformer function you pass in.
If transformation produces an optional value, then you would just receive an array of non-optional values.
let possibleNumbers = ["1", "2", "three", "///4///", "5"]
let mapped = possibleNumbers.map({ Int($0) })
// [1, 2, nil, nil, 5]
let flatMapped = possibleNumbers.flatMap({ Int($0) })
// [1, 2, 5]
if transformation produces a sequence or collection for each element, then you would receive a single-level collection.
let numbers = [1, 2, 3, 4]
let mapped = numbers.map { Array(repeating: $0, count: $0) }
// [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]]
let flatMapped = numbers.flatMap { Array(repeating: $0, count: $0) }
// [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]