@ LppEdd - спасибо, я понял это
var arrOne:MutableList<Int> = mutableListOf<Int>(3) //will result in [3]
//add() extends the list
arrOne.add(11) //[3, 11]
arrOne.add(43) //[3, 11, 43]
arrOne.add(35) //[3, 11, 43, 35]
arrOne.add(47) //[3, 11, 43, 35, 47]
//set() replace the value at the given index
arrOne.set(0, 23) //[23, 11, 43, 35, 47]
arrOne.set(1, 34) //[23, 34, 43, 35, 47]
arrOne.set(2, 56) //[23, 34, 56, 35, 47]
arrOne.set(3, 66) //[23, 34, 56, 66, 47]