Splice() and spread operator (...)

JavaScript
3

They don’t talk about … (spread).
furthermore they give as example only arrays with strings inside.

But that’s works now.
This my solution:

function frankenSplice(arr1, arr2, n) {
  let copyArr2 = arr2.slice(0);
  copyArr2.splice(n, 0, ...arr1);
  return copyArr2;
}

frankenSplice([1, 2, 3], [4, 5, 6], 1);

I made the same mistake like in this other topic.
https://forum.freecodecamp.org/t/title-case-a-sentence-cascading-methodes/476760/3
I’m still really not well understand with that.

If I write this code, that not works but why ?

function frankenSplice(arr1, arr2, n) {
  let copyArr2 = arr2.slice(0);
  let result = copyArr2.splice(n, 0, ...arr1);
  return result;
}

frankenSplice([1, 2, 3], [4, 5, 6], 1);