6.1 Naive version (lower difficulty)

type NaiveFlat<T extends any[]> = {
  [K in keyof T]: T[K] extends any[] ? T[K][number] : T[K]
}[number];

The solution of naive version is mapped type which traverse through type T given as argument. Important parts:

• T[K] extends any[] - question if value type of property K is an array
• T[K] extends any[] ? T[K][number] : T[K] - if T[K] is an array we get its value type by indexed type [number]. number is correct key of T[K] as T[K] was checked before for being an array, and array has number keys
• [number] at the end has a purpose of getting all value types of produced type

6.2 Deep version (higher difficulty)

type DeepFlat<T extends any[]> = {
  [K in keyof T]: T[K] extends any[] ? DeepFlat<T[K]> : T[K]
}[number]

The solution of deep version is different only in one point, instead of T[K][number] we go further and call DeepFlat<T[K]> so its recursive type, it calls itself. Thanks to the recursive nature we traverse the object until value type is not an array what is visible in "else" part : T[K]

The full code with test suite can be found in The Playground

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