Comment by granzymes

For one, the simple answer is incomplete. It gives the fully unwrapped type of the array but you still need something like

  type FlatArray<T extends unknown[]> = Flatten<T[number]>[]

The main difference is that the first, rest logic in the complex version lets you maintain information TypeScript has about the length/positional types of the array. After flattening a 3-tuple of a number, boolean, and string array TypeScript can remember that the first index is a number, the second index is a boolean, and the remaining indices are strings. The second version of the type will give each index the type number | boolean | string.